I can't answer about specific vmware benchmarks - but a strong possibility is better graphics drivers with 10.04 compared to 8.04.
You may have been having 3D graphics issues with 8.04 - this is likely to have improved under lucid and thus the full 3D graphics capabilities in Workstation 6.5 would now be available.
In addition - if you did a new 10.04 install with a ext4 filesystem you will have noticed a better disk-throughput compared to the ext3 filesystem in 8.04. This was discussed and benchmarked by Phoronix when comparing KVM benchmarks from 8.04 through to 10.10.
Without knowing what hardware you have, its a little difficult to give you specifics.