I see what your saying about it being sensitive with such a big WG, hadn't thought about that!
But it depends how much gasses the rotary produces, if they flow large amounts (more than the turbo will ever need to spool to the desired boost) then a big waste gate is needed, if on the other hand the GT42 needs say 90% of what the engine flows then a smaller gate will be needed....
A 60mm WG has been recommended by a tuner that has built many big power RX7's so I can only assume the advice is correct.