Bigger would be nice, as the twin pass cores are never as efficient, but I'm sure with Meth it'd be OK.
However,
You'll need to do some tweaking to the factory charge pipe at the back of the bay though. The ko4 sits much higher up, and if you're using a relentless manifold (it's pointless doing the conversion if you don't) then it'll stop the oem charge pipes running across the back of the bay.
to be honest with you mate, when the time comes I think you'd be better off selling the THS cooler for good money and fitting a toyosports type B core, as it'd work out about the same price wise in the end, and do a better job.