Forget 8ms, most are now 5ms which is much better, and there's a few 4ms out there.
Performance hit depends -
If your CPU limited, probably not, the extra "size" will be done by the GPU.
Just fitted a 22" LCD to mine yesterday, very happy with it.
At the moment all the 22" widescreen monitors use the same LCD screens, so you can base your choice on what else comes with it.