Comparing Mhz from 2005 to 2011

i know i may asking for the holy grail here, but i'd like to share some thoughts on that.

a couple of months ago i was planning an hw refresh for a farm built way back in 2005, based on Xeon MP 3.0 Ghz processors, with newer blades with some X5660 processors.
looking at the vcenter statistics i can see the actual farm is using something like 50Ghz as an average, and has a physical to virtual cpu ratio of about 1.5.
by a rule of thumb, i may say that on newer processors, doubling the p2v cpu ratio won't cause any problem, even if i don't have any evidence to prove it.
speaking of Mhz instead, i'm trying to find out how a 2011 core compares to a 2005 one.
even if X5660 cores are running at 2.8Ghz, i don't think they're "slower" than Xeon MP ones.
i've fiddled a litte with the published spec.org CINT2006 benchmark results
here is a sample:

 xeon 3.8Ghz        11
e5345 2.33Ghz       16

e7330 2.4Ghz        17

e7458 2.4Ghz        20
x5460 3.16Ghz       27

x5560 2.8Ghz        35

so, it seems that the X5560 is at least 3 times faster than the old xeon.

after some weeks, i've stumbled upon this blog post that seems to come to the same conclusion
http://it20.info/2011/06/the-cloud-and-the-sunset-of-the-ghz-based-cpu-metric/
MIPS have served well for the mainframe world, why shouldn't they work for x86 also?