200hp will last several race seasons if you don't do burnouts on Sydney roads. So <12psi boost on the sr20det will be fine. It's depends on what your definition of 'taking it easy is' .

Sydney roads are full of potholes and dangerous bumps, which are diff's enemy.

I'd recommend you doing the R31 now though, because it will mean having a second tailshaft made up if you do it later.