Frequency is only one aspect. And you're right... isn't grow due to some physical/thermal limits.
But you have consider also the cache (L1, L2 and L3), core architectures, bus speeds, ...
Platespin Recon also normalized the Mhz to the latest technology ..
i've fiddled a litte with the published spec.org CINT2006 benchmark results
here is a sample
xeon 3.8Ghz 11.4
e7330 2.4Ghz 17
e7458 2.4Ghz 20
x5560 2.8Ghz 35
Based on your 50GHz and 1.5 vCPU/pCPU ratio, I'm going to take a WAG and say that'll drop to something like 18GHz. You will see an absolutely incredible amount of performance gain.
Normalizing those SPECint figures doesn't mean much because it assumes a linear scale up factor. Newer processors are much more about scale-out: cache speed, core count, memory speed, die size etc. All of these factor into why a slower CPU can still be way faster.
So while it appears the higher end CPU is only about 3 times faster - thats only on a single thread that it is 3 times faster. Consider your Xeon MP 3.0GHz was only capable of processing 4 threads at once, a x5660 will do 12+