are you running the map sensor off of two intake runners or one? if you are only running it off of one of them you need to change it ie use runners 3&4 (after the throttle plates) as it uses individual throttles and the runners are so short you wont get a stable map signal at low throttle on just one runner = shit low down drivablility.
while its possible to make the power you are after with that turbo you might as well take the turbo off completely and not bother replacing it as it wont come on boost untill 5000+rpm before losing all traction and needing to change gear.
a 0.6x exhaust housing on a big turbo is still massive for a little RB20.
Read this and it will explain a lot to you
http://www.hotrod.com/techarticles/engine/113_0312_turbo_tech_science_selection/compressor_turbine_housing.html
Everyone i have talked to has said you need at minimum a 100mm thick intercooler with 3" in and out.
OEM RB26 intake is fine as is
Valve springs may be an issue, see if you start to get float during tuning or change them now if you are worried
its also still not P plater legal as its then a modified car even if its not turbo anymore.
it will have all the guts of a suzuki alto with none of the sub 1 ton advantages.
im not sure what the legalities would be though as a cat doesnt work unless its hot and a screamer doesnt have hot exhaust running through it all the time.
i have seen it done, a guy put a muffler on his screamer pipe a few years ago.
http://www.skylinesaustralia.com/forums/topic/209139-quiet-screamer-pipe
it doesnt work that way. octane doesnt change the volume required. E85 physically requires 30% or so more fuel for the given power vs petrol. running 98 on an e85 tune is going to be driving around at 10:1 or so (fubar the plugs in 5-10 mins)
you will not find massive difference in the octane rating from E70 to E85 in terms of detonation tune for E85 and drive on it year round be it from E70 or E85.