Hi! Few days ago I've changed my ROM from Energy to 826x Series(from 18/10/2010). Xperia seems to be running smooth at stock speed, but this ROM allowed me to OC my device up to 768Mhz(tested with sk tools, it is really that speed). It's great, no SOD or any other glitches(With energy my x1 had SODs even at ~650-670Mhz).
@
fil--21, let me answer

Firstly, install MSM7kCpuSpeed to your device memory. Then install SetCpuSpeed to your sd card(when it asks you to choose speed and intervals click "cancel", it will install anyway). Then launch your favourite registry editor, go to HKLM -> Software -> Msm7kcpuSpeed and there set following parameters:
BootTime = 70000 (it is time in milisecods, 70000 = 70 seconds, you can increase this value to 90 000 if you meet any hangs at boot). you shouldn't go below that value -> it is good to let your xperia fully boot, and that change cpu clock.
CheckTime -> you can experiment with it. around 20 is fine(I'm currently using 17)
PerformanceLevel - 40 (40 means 768Mhz, every step = 19.2Mhz)
There are the only values you should change. Of course, after every boot you should wait those ~ 2 minutes before using phone(just to make sure you won't get sod).
BTW, I'm using v1 version of Msm7kCpuSpeed, it seems to cause less sods and other glitches(for me, at least).