We have a Poweredge 2950 running Hyper-V with a six disk RAID5 and there are no performance problems. This only runs ten VMs, so you could argue that it's not that heavily loaded, all the same the disk performance is very good. Very, very good in fact!
People seem to have a real downer on RAID5, and it's certainly true that if you have the array configured for write-through the write performance is poor. However the Perc 5/i and 6/i controllers seem to manage write caching very effectively and unless your system is doing an awful lot of writes I doubt disk speed would be a problem. Yes, RAID10 will be faster, but that isn't the same as saying that RAID5 is too slow.