Agreed. The GT35 will make the power you are after, however it is definetely not something i would call "super responsive that comes on boost soon". Problem is for 350rwkw your looking at a 700HP turbo which is something that flows around 1000cfm at over 1.3bar. On a 2.5/2.6L this means 4000rpm+ to start seeing a nice midrange. Look at compressor maps and peoples dyno graphs, dont look at peak power, but area under the curve and at what rpm different setups make diferent power, than judge for yourself how "responsive" you would call that.
One thing i have learnt over the years from these forums is people judge response very differently. One's persons responsive turbo is another persons lag monster.
Oh yeah forget about doing the single RB25 GTRS, would be more worthwhile keeping the factory twins.