making an itb setup work is not hard. having machining equipment, a super flow 600 flow bench, serdi head machine, and an engine dyno doesnt hurt either though
they hardest part of an itb setup is tuning it. most likely you will not be able to run your stock ecu and maf. well you can but i would rather go with a nice standalone like electromotive. high resolution and very strong ignition as well as tons of ecu functions to play with.
this is my current project since the ITB's on my sr20 are done.
i got a set of toyota atlantic 4age TRD itb's. they come in pair of two. im doing this for my 1jz-gte head. i got the whole itb setup for 75 bucks
i did all the measurements and already made the adapter flange drawing in Pro-E.
the amount of R&D work that goes into actual racing engines is insane and let not talk about the cost. the only solution is to do it yourself. there are plenty of engineering and non engineering book you can find on fluid flow.
you can get really technical with it and pick up Sengel's books on thermo, heat transfer, and fluids. or you can pick up an old school book called "practical gas flow"
both have helped me greatly to understand how to port heads and why it is done in certain areas. after doing some light math i can see what they are talking about. finally the work is backed up testing on a flow bench.
for me being a 23 year old college student i can afford to buy bolt on itb setups but i have some time on my hands and access to tools to learn to do it myself.
btw the gti-r setup uses regular top feed low impedance injectors. they are 444cc stock and are yellow. they are the same type injector as the RB-series engines(not sure of fuel flow rating on the rb injectors though)