Well I tried using that xu_killmobs and it didn't help at all. Around 92 mobs were killed though. Didn't manage to try xu_killliving as my client froze shortly after when I tried accessing my crafting table multiple times. I got a feeling it's not a fps issue. It seems to be more of a performance issue at night when I notice a bit of delay opening a crafting table for example. The longer into the first night, the longer the client freezes last and ultimately just almost lock up the client completely. My fps dropping is probably a side effect of this but I ain't 100% sure, hard to tell.
Never seen this happen on other mod packs but I never played a pack before with a lot of mobs roaming around. I've played FTB Ultimate while even running a server on my own machine with 2 players. If I'm not wrong that pack is pretty heavy too.
On a side note:
CPU: i5-2500K 3.3GHz
GPU: AMD 6850HD
Memory: 8GB total (no clue which or speed)
On the FTB launcher I got the memory slider at 5,75GB.
Anyways, waiting for 1.6.0 or whichever comes next.
/edit 1
After rebooting my client and leaving my hideout cave at daytime everything was just fine again. Let's see what happens in the next night.