Man, I'd love to accidently buy an 8 core ES CPU.
Unless it was some 'cherry' ES, I'm not sure it would be all that much better, unless you are talking about IB-E.
Just conjecture here, but say you could get a SB-E quad to 4.0ghz with 8 cores; thats not a LOT faster than 6 @ 4.6ghz. You could get a bump is very highly-threaded applications though, maybe 10-20%?
IB-E would be totally different. With potential reduced power consumption, you could (just conjecture here) get 8 cores to 4.6ghz, where you could only get 6 cores before. That could be pretty slick. 25% more 'raw' power + an additional 5-10% IPC would yield a pretty nice bump.
I know Aigo will pipe-in eventually and say '8s not enough, I want 10!'. 10 cores at ~4ghz with the IB IPC improvements would be EVEN better for highly-threaded apps.
It really comes down to balacing your needs for threads and clockspeed. Right now, SB-E gives you 50% more cores for only ~10% less clockspeed. Thats a good trade-off for what I use it for. For others, the reduction in clockspeed may not be worth the additional threads.