"Only" 320 rwkw here and I'm up to 332,000km on my XR6T. Not bad! Cost me about $5100 including cooler, injectors, fuel pump, brand new full exhaust, labour and the $600 XCAL tune box, back in 2010.
$13k on a stock FG F6 and I'd be expecting 400rwkw++ for sure on 98. Up to 500rwkw on E85 I would think?
I want a boosted V8 for a weekender though. That sweet noise.