Just thought I would add my results to this. I was having performance issues with Microserver, although apparently not as severe as some of you were having. I decided to run a basic benchmark before and after changing the drivers. Results were quite dramatic and I could tell as soon as I booted the VM that the performance had changed.

I ran Crystal Disk Mark (all default settings) and the output is as follows:

ESXi 5.5.0 U3a

Before (v100 drivers):

After (v88 drivers):

Don't really understand the significance of the numbers that Crystal Disk Produces, but the numbers seem a lot higher!

Thanks for the help all.