We were receiving reports of some developers experiencing a degradation in JVM compile performance and general compiled application performance. We had just recently vMotion'd these VM's from HP G7 systems w/ Westmere processors to newer Dell M620 blades running Sandy Bridge Processors.
Long story short, applications compiled on Westmere don't take advantage of a new AVX CPU extension available on Sandy Bridge, which in turn is only made available in RHEL 6.2 and newer kernels. I'm not sure if Windows would be affected in the same way, but this was a definite issue in RHEL 5.8.
The performance delta was not 'huge' but enough in terms of scale.