Even in MWLL it will matter. Clock to clock it's slower. Then factor in it's only two cores, MWLL/Crysis can use three cores if you set physics to another core. Not to mention maxing two cores when windows wants to use some processing power = no good. While all cores on the X6 aren't getting used in MWLL, the CPU itself is faster and better.
It's the same with Second and first gen i7. They are both quad cores, both have HT, but sandy beats the crap out of first gen i7 for playing MWLL.
My GC get bottlenecked in MWLL on one screen even with only ONE graphic card, and I have sandy! MWLL needs as much CPU power as you can throw at it.
If we bothered with stock clocks around here, then sure, it's a whole 100 mHz faster.
The 550/555/560/565 are all Deneb X4 cores with two disabled through binning. The only thing the Thuban architecture (which is exactly the same as Opteron Instanbul with a different name for the desktop market) has over the Deneb is it's "T" (Turbo) designation, which is the same as Turbo Boost on Intel i7; an effective clock increase when only one or two cores are under load. I'm not saying a dinky X2 is as good as an X6, just that in Crysis, you will not notice a difference, regardless of which core you assign the physics thread. I've run benchmarks in MWLL on a 550 X2 and the same X2 unlocked to a X4, both on stock clocks and at 3.8 GHz (2200 HyperTransport), and there is literally no difference
Strictly speaking, the Deneb X4 will beat a 1090T in Crysis with either a single GPU or CF/SLI setup, either because they both share the same 6MB L3 cache across all cores, or because the 1090T actually has a lower stock clock speed, take your pick...there's a Tom's Hardware article on it somewhere, but I'm too lazy to find it. An X4 Zosma (quad-core version of Thuban/Instanbul) will likely outperform it as well.