I loved the various extensions available for GNOME Shell but I don't want to enjoy their benefits at the cost of slowing the system down. Is there any known impact?
Newer systems will not notice a dip in overall speed. The main effect they will have will be on the RAM, where everything is loaded, but even that will depend on what type of extension you're talking about. For example, the binary clock will have almost no effect on the CPU usage (processing speed) and very little effect on the RAM.
So I guess my answer is, No.
You can use the System Monitor to ensure things are not getting out of hand with gnome-shell.