a few myths here.
firstly, twins or single, if they are sized comparativley, there will be NO noticeable difference in spool OR whatever. that is a myth brought about due to manufacturers using staged twin setups.
just size correctly and if you can get the turbos u need for twin cheaper than singel, do it. same end result.
everyone cries custom intercooler. just get hold of a garret truck IC and have it modified to suit. considering they are utilised on 12+ L trucks that pump more than 20 psi from just of idle, cost 50K + for a rebuild, and the difference in power afforded by a efficiant IC means fuel economy saved by less throttle load (im a truckie, so talking from experience)
"Add forged pistons",
Just as an aside note, it was mentioned in street machine that jake edwards turbo 308 torana was runnign CAST pistons. and a realativley stock bottom end ( stronger rods and bolts) so it isn't always necessary to run forgies, especially in this setup which will be custom = $$$$, and as such it isnt cylnder ressure that causes pistons to fail, its TUNING (eg ping) so tune right
While this can aid in sealing in combustion and etc, having the head gasket as the weak link (relativley speaking) can save you from having a costly rebuild when something has to let go. its so much easier to let the gasket go, than pushing a piston and rod down thru the crank.
" ported head "
apart from match morting and smothing out the ports to remove and obstructions, you dont need to go very far AT all with porting. cause after all, the turbo(S) will be forcing air thru. so as long at the parthways are smooth, its all good.
"6 throttle body setup"
NO WAY would you want this
the extra flow restrictions caused by the 6 throttle plates is un necessary a large single will do the job quite well, and has been proven to see times. why would the jap guy pull the 6 TB setups of the extreme GT-R's if they worked better?
"high flowing injectors"
more like fire hoses
if its going to be a drag only car then a 4" dump pipe straight to the ground is all u need, witha seperate screamer pipe for the wastegate.
when u hit the t8834D ( i think thats amongst the largest) you NEED to goto twins as there simply arent any competition turbos available to go better. hence the cars coming out using twin t72's