Only 32-bit WIndows XP and 32-bit Windows 2003 (without SP2) benefit from FlexPriority. The performance impact of disabling FlexPriority varies depending on workload, but typically it will be negligible. If your workload is extremely sensitive to this feature, you may find that you get the best performance by using binary translation instead of Intel VT-x.
You will only be affected by this bug if you are running 32-bit Windows VMs with PAE (physical address extensions) enabled.