Well I was going to point you to How to know if hard drive supports Sata3 (6Gb/s) but your question is not what Sata version is supported but if the 400 you are seeing are in fact Sata3.
TIPS WHEN SELECTING A GREAT SSD DRIVE
I actually made a benchmark test with 3 hard drives on the same motherboard found here in Spanish and the things I learned from it were:
Not all SSD companies tell you the truth. I don't mean the "in theory it should achieve speeds of..." truth. I mean "I tested this and all I got was this lousy T-shirt". With this I mean that, one thing is what they tell you the SSD can achieve, another is what it actually achieves.
Not all SSD models from the same company achieve the same speeds. Or worse, maintain the same speeds for months.
Your favorite SSD company might actually be worse than that other new weirdly named company.
The motherboard can actually have a huge impact on the SSD's performance.
BENCHMARK (Samsung vs Intel vs Kingston)
Here is the information I gathered while I tested the following models (Noticed I changed the notation to BYTES, not BITS since we want the real world truth):
Intel 520 Series 120GB SSD which mentions in theory a READ speed of 550 MB/s and a WRITE speed of 500 MB/s.
Kingston SSDNow V300 120GB which mentions in theory a READ speed of 525 MB/s and a WRITE speed of 500 MB/s.
Samsung PRO 256GB which mentions in theory a READ speed of 540MB/s and a WRITE speed of 520 MB/s.
I got the following results from 10 tests with the Disks benchmark tool and 3 more with hdparm:
For the Intel, it showed an average of 465 MB/s (READ) & 329 MB/s (WRITE) with the Disks Benchmark. With hdparm I got an average of 398 MB/s (READ).
For Kingston which was the one I assumed would be the best of all 3 got an average of 404 MB/s (READ) & 298 MB/s (WRITE) with the Disks Benchmark. With hdparm I got 376 MB/s (READ).
For Samsung, the Disk Benchmark had a problem doing the Write test, so I could only do the READ test which gave me a 555MB/s. hdparm gave me 519MB/s (READ). Now, since I don't like to leave things half-baked, I did a normal test (10 times) of copying a file from one place to another on the same SSD. I first tried 1GB but it was too fast to test and find an average. I then tested 2GB, then 5GB and at last got to 10GB (Or 10.2GB to be exact) to test this. The average WRITE speed landed at 753MB/s.
So My first thought of seeing the Kingston as the fastest was wrong. Even my second thought of the Intel being the fastest was wrong. It was Samsung. Now I did the test right now again and got a speed of 716MB/s (WRITE). The drop might be X variable on the system but the speed nevertheless is very high.
BENCHMARK (Changing Motherboards)
Here are the impacts if I change motherboard (The previous test was done on an Asus Z87 PRO updated with the latest firmware). If I jump to an Intel DZ68DB which also advertises Sata3 (6.0Gb/s), all SSD drop between 40% to 60% of their speed with the Asus). This is how important the motherboard is. If you have a low cost motherboard with a high cost SSD, well, you just wasted your money on the SSD because of the limiting factor the motherboard will have on the SSD. To give you an idea, the Samsung was doing a 265MB/s READ speed and a 212MB/s WRITE on the DZ68DB. That's a huge drop.
Now here is an update for the ASRock H61M/U3S3 & the Gigabyte GA-H77M-D3H.
With the ASRock using the Disk utility 10 times, the Intel SSD got 140MB/s (READ) & 110MB/s (WRITE). the Samsung SSD got 180MB/s (READ) & 123MB/s (WRITE). I don't have the Kingston right now but you can guess from the other two how it would go with it.
With the Gigabyte using the Disk utility 10 times, the Intel SSD got 241MB/s (READ) & 209MB/s (WRITE). the Samsung SSD got 312MB/s (READ) & 295MB/s (WRITE). Again I don't have the Kingston right now but you can guess from the other two how it would go with it.
On all cases, hdparm, dmidecode, dmesg and any other HDD/SSD reading tool showed that I was "capable" of using Sata3, but the actual READ/WRITE speeds where not met to the speed one assumes to get from using Sata3 because it varies greatly between motherboards, SSD models and any other variable in between.
With all of this said and done, to answer your question: Is 400MB/s a Sata3 speed?
Yes, yes it is. Sata2 would really limit you from around 180Mb/s to 250MB/s in a real world scenario. Sata3 would push the boundaries well above the 300MB/s. But this is also affected by the motherboard, SSD model & SSD manufacturer.
On Hard Drive Benchmark which I always check to see which SSD to buy or recommend I found that your Crucial MX100 has 3 types of sizes, of which, only 2 appear in that list, the 512 & 256 versions. Each one with their scores. So here are the scores of the benchmarked SSDs I tested with the 3 types:
- Intel 520 Series 120GB - 3699
- Kingston SSDNow V300 120GB - 2514
- Samsung PRO 256GB - 3536
- Crucial MX100 256GB - 2916
- Crucial MX100 256GB - 3729 (Model 2)
- Crucial MX100 512GB - 3480
From the scores one would think the best is the Model 2 of Crucial, followed by Intel, but one would be wrong. Factors like motherboard, firmware version (For both, mobo and SSD), operating system and even BIOS/UEFI settings can change everything.
For my current case, enabling TRIM on the Samsung gave me a boost of about 150MB, which lands me in the 500+ Speeds. I actually did not know TRIM could change the speed until I enabled it. Another factor was updating the Asus firmware.
So yes, you are using Sata3, especially with a 365MB/s speed but something (Maybe motherboard, firmware, the actual SSD, the SSD cable, the geographical location, the altitude, Santa claus, some weird freaky alien electrical monster living inside your computer case, etc....) can have an effect on the actual performance. BTW, some motherboards when using old hardware connected to it, can somehow limit the performance of the rest. For example, having an old PATA CD-ROM or DVD-ROM connected can have a negative performance on the drive. Same goes for a 3.5 Floppy drive (I mean come on O.o).
Updating Ubuntu to a more recent version can have most of the time a positive impact on performance, especially if you read the performance improvements on the 3.14, 3.15 and 3.16 kernels.