If all your VMs are sharing the same disks you'll get better overall throughput because your disk controller (and the disks themselves) should be able to reorder reads and writes. With a larger buffer depth, it will be able to reorder those reads and writes better. If your concern is that one VM will be doing a ton of disk access and slow down the other ones...well, you just need a hypervisor that supports quality of service and makes sure one VM doesn't starve another one (offhand, I think Solaris Containers allows this).
For the disk configuration, it depends on your workload. RAID5 performs badly if you're doing a lot of random writes (e.g. database). For sequential writes, or a workload that is mostly reads, you'll get better performance out of it. But the main downside to RAID5 arrays is their fragility - two disks fail and you lose everything. And those double failures happen more often than you'd think. Overall if you can afford it, RAID10 offers a better balance of performance and reliability.