Pretty sure the R200 diff is good to 600hp, even though the axles aren't. Not sure what the capacity of the KA /SR transmission is (they
are the same, cept for bellhousing, right? Same for RB20, ya?), but I think it's in the area of 350-450 hp. The only internal upgrade I've ever seen for a KA / SR is made by OS GIKEN, and runs around $4k ish. Enjuku has it
http://www.enjukuracing.com/products/OS ... hafts.html
+1 on the upgraded clutch and mounts. You *might* be snapping axles at over 400hp if you drop the hammer too hard. Not sure on that one.
Someone more knowledgeable than me (not difficult) could probably give you more info.