Would this allow me to more easilly transfer it to faster hardware in the future?
Sure.
Or would running as a guest seriously hurt disk i/o?
Depends on what you use for IO :) It's not impossible to replace the physical iron with a virtualized iron and config that's faster.
What do you have available, 10g eth for example? That will dictate the likely success.
I had a few platforms exporting lvm backed block devices (and iscsi once) and the differences between a non BBUWC LSI card on sata's compared to an HP P800 no SAS was like 10:1, and a P800 is by no means a screamer but it moves an exchange server, sql, and a few other vm's along fast.
You might also compose the raid array appropriately for best results. Search here or the iet list for Ross Walkers suggestions, I have used his advice with good success.
jlc