Roy on here has some 260 odd kW at the rears. i personally havn't heard of much drastically higher than that.
rather than spending dollars on making sure the engine is healthy up at the high kW range, people would rather go with the RB25 or RB26 as it's easier to acheive. though, it would be cool to see an RB20 hit the big 300.
the lower compression engine would see some bad lag with a monstrous turbo.