This is precisely why I'm recommending the Xperia L because it does not get nearly as hot as the galaxy s2 or s(i9000). Do a little research on the chipset of the Xperia L and you will find tons of useful information about potential and efficiency of krait architecture. Krait is superior in every way.
As for the custom Rom and kernal in the s2 believe me I have installed tons of custom roms on my s2 and they are just not stable enough there is always a bug that bites you in the ass
and same goes for custom kernals. The only custom roms and kernals that are worth using on the S2 at the moment are those that are stock based.
I admit that custom roms are great fun but beyond that they are not practical because they will always have bugs and because they are created by novice to intermediate developers and not professional developers that work in HTC ,samsung,Sony wagera,wagera
If you visit xda regularly then you'd know that most of the developers in that forum still go to school means they are not professional developers and testers.