Well,Usman! BOVs,apart from making shrilling noise they reduce the turbo lag, this is a basic purpose of a BOV, i dont know any "desi tareeqa" of making a BOV, it just holds pressure for a while and makes turbo spin quickly then release that pressure, i dont think that BOV helps increasing pressure of turbo
Well if u wanna check whether ur turbo is water cooled/oil cooled then u must look for water lines (pipes flowing from radiator to ur turbo) if u see pipes! then ur turbo is watercooled otherwise Oil cooled
Touching 150bhp is easy in GTti, remove that dump pipe which is in between ur exhausht pipe and turbo with a stand free flow pipe, add a muffler, install intake if u really want to then go for a BOV too after that ur GT will b around 130bhp/125bhp. change the intercooler pipe from that rubber pipe to a heavier one, install plugs+plug wires for better current and sparks.
rather then changing ur ECU/Turbo install a valve between ur boost control solinoid and actuator to increase boost to 13 or 12.5 psi(each PSI incearse is around 8/9bhp increase in power) so that means if u increase 4psi then bhp increase will b around 30/32bhp ..... see u've achieved 150bhp mark 
i've done this already 
and everything custom by myself though im no automobile engineer/mechanical engineer 
adios! go for all these things and do post pics of ur GTxx + 300ZX nissan i love that car, same goes for u tauseef.