Sorry to re-post the data but I wanted to draw some conclusions from it:

Code:
Ubuntu Desktop 2.6.27-9-generic SMP x64, compiling kernel version 2.6.28.1
make menuconfig, saved without changing settings.
ReiserFS on a Seagate 7200.11 1TB drive

time make -j8 vmlinux, fresh boot, text mode, 3-6 back to back runs

best real times used:

3.8ghz, 3200mhz uncore: real 1m2.207s/user 5m53.758s/sys 0m42.675s
3.8ghz, 3400mhz uncore: real 1m1.152s/user 5m51.986s/sys 0m42.591s
4.0ghz, 3200mhz uncore: real 0m58.972s/user 5m37.037s/sys 0m42.019s
4.0ghz, 3400mhz uncore: real 0m56.923s/user 5m36.913s/sys 0m41.127s
4.0ghz, 3600mhz uncore: real 0m56.887s/user 5m33.673s/sys 0m41.039s
4.2ghz, 3200mhz uncore: real 0m55.090s/user 5m23.288s/sys 0m41.315s
4.2ghz, 3400mhz uncore: real 0m54.617s/user 5m22.072s/sys 0m39.890s
4.2ghz, 3600mhz uncore: real 0m54.440s/user 5m19.956s/sys 0m39.370s
Looks like you need at least 3400mhz uncore to not hold back the cores at 4.0ghz.

Going from 3200mhz to 3400mhz uncore @ 3.8ghz is a small gain but it seems more stable on my system so I'm using it.

Uncore gains @ 4.2ghz are relatively small so feel free leaving it low to be stable.