looking at a 98s with 4k on new 3.8 top end, Protomotive tune, 90k on low end. What should I expect a rear wheels (91 oct) conservatively?

 

Well, 3.8 adds 5% increase in displacement, so if the efficiency hasn't changed, you can expect a 5% increase in HP. My guess is that a Protomotive tune can, at best, extract another 25 HP. S if you start with 282 Engine HP, you may get as much as 320 or so to the flywheel. Your question is about RW HP, and that is more difficult, as each dyno seems to produce different results.

 

If you use 15% drive train loss, you may see 270 at the wheels.

 

The Mahle P&C conversion was worth 35 hp at the flywheel but they bumped the compression a few tenths in the process.
What was more impressive was the big increase in torque which helped coming out of the corners on the track. It was the power under the curve that made such a difference.