This has been discussed many times in various forms, most of which I've been an 'Outside Watcher' not participating, just observing. Although OC and I, along with others have batted this about a few times...

Mainly because I don't think a meaningful comparison can be drawn unless the various CPU's and versions can be monitored running the same science/project. Given the different complexities and lengths of runtime of the different projects work units etc...

Personally, I strongly believe the newer processors produce a better output and with the bonus of lower power consumption.

A lot of grid crunching comes down to what hardware is available/affordable to the individual.

There are so many variables, from the amount of RAM to the HDD/SSD the system is running on, to say nothing of any other tasks/usage...

Something our earlier discussions appear to have confirmed is that the more threads a machine runs, the more economical the system is on power. Mainly down to the reduced peripherals, i.e. HDD/PSU/MB/RAM etc. so 2 x 8 thread systems would cost more to run than a single, equivalent single 16 thread machine with similar peripherals.

While this is a little 'Non-committal' hopefully it does show the complexities of comparison.

It is only my opinion/conclusions from my crunching and discussions as mentioned...

The main thing that seems to matter is the manufacturing technology. At 22nm and below there isn't much performance difference. Signaling the end of Moore's Law. Newer Xeons may use less electricity but it isn't that significant.

Edit:
At 22nm Intel intoduced another ALU and AGU to each core, which provides a significant advantage over 32nm Sandy Bridge. Since then not much has happened to increase Instructions Per Clockcycle. AMD is introducing their 14nm Zen processor soon in 8+ thread versions. It may not be as efficient as Xeons, but they are pricing lots of threads at a relatively inexpensive price compared to Xeons. I've heard a 40% increase in IPC but we shall see.

So the difference between an E5-2699 V4 (22 cores / 2.2 GHz) and an E5-2698 V4 (20 cores / 2.2 GHz) at 17% from 32248 to 37929 is more than the 10% difference in cores?
Strange.

And in the google spreadsheat in the second post, a dual E5-2695 ES V4 @ 2.8 GHz differs from a dual E5-2683 ES V3 @ 2.3 GHz by exactly the difference in the clockspeed, running the same OS and the same tasks - which implies v3 to v4 is no upgrade at all.

If I compute the google spreadsheet and divide Boinc PPD's mixed units by the number of cores and the clockspeed, to get a PPD per core per GHz - the highest number is from a 2x Xeon E5450 @ 3 GHz, which comes in at 324 PPD/core/GHz (7777 PPD for 8 cores) compared to an E5-2628L V4 @ 1.5 GHz which comes in at 277 PPD/core/GHz (10002 PPD for 24 cores) - both running linux.

If I remove the windows machines, I get something like

v0 (socket 1366 and earlier) 230 PPD/core/GHz
v1 256 PPD/core/GHz
v2 265 PPD/core/GHz
v3 264 PPD/core/GHz
v4 280 PPD/core/GHz

I'm no wiser than when I started this....

I used the column BOINC PPD mixed wu's - which leaves out wattage. Still, it appears replacing a quad Opteron 6282 which is doing about 30k PPD is hard (only the very best cpu's are faster).