They are RPM limited because of only having 4 main bearings. And a ridiculously long stroke (by modern standards). A Supra 2JZ-GTE engine with a 86mm stroke can rev safely to 7500+ rpm without hitting piston speed limits. A 225 slant with it's 104.7mm stroke hits the limit at under 6500 rpm.
They are boost limited by having only four smallish head studs per cylinder. They are breathing limited by the non-crossflow head.
There are 650 hp turbo slants around, but not many.
The OP has a very nice setup, well thought out.
