1x Hypervisor. 4x VMs. All on single array RAID 5 (4x disks SATA 7.2K). Need a performance boost. 5 users.

Performance improvement suggestions.

Quick scenario. 1U server. 4x VMs. All across 4x Drives in RAID5 7.2K SATA. Total 10TB. Used 4TB
VMs are growing and not committed. Single Array split into C:OS and D:Data.
NON are dynamic RAM.
96GB DDR3 1333
16x cores (1.8Ghz 6MB Cache I think).

RDP runs on one VM and the handful of users (5) are reporting sluggish performance with peaks and troughs.
All 4x VMs are running on 1 VM Nic and another Nic for host.
Reason for this setup is all but 1 user is connecting via the Web. so no network traffic. So no need for lots of VM Nics or a team IMO.
No mega budget. Maybe £500. They do not want to run 2 hosts. Happy with vCPUs for the VMs.

Initial ideas:

Low impact:
Commit the VM OS disks to full/thick as opposed to grow/thin provision.

High impact:
Only 4x drive bays. HARD WAY - move VM OS and VM Data and Data on host to second server temporarily or NAS BOX with ISCSI Target as these are cheap.
Swap 2 drives for SAS 15K (RAID1) or even better SSD, but prob cost prohibitive. Last 2 drives make a RAID1 for data as the slower storage is fine. Having only 4TB usable, might cut it. Or need to get a cheap NAS for slow and VERY infrequent access.

Any other ideas, or better solution to moving the data off if drive swap is enough.

The server is good enough. Theres enough RAM. So no more IMHO.

Thanks in advance everyone.

R