We put T2000 (8x core, 1GHz) instead of E6500 with 12x US-II 400MHz into our production. We've got heavily multithreaded applications here. Server is doing quite a lot of small NFS transactions and some basic data processing. We didn't recompile applications for T1 - we used the same binaries as for US-II. Applications do use FPU rarely.
T2000 gives as about 5-7x the performance of E6500 in that environment.
Well, "quite" good I would say :)
ps. probably we can squeeze even more from T2000. Right now 'coz lack of time we stay with 5-7x.