Firstly, I run a kit in my 1.9CDTI Vectra which has been remapped and modified to death.
I have two, yes TWO intercoolers, both of which would make most cars radiators look inadequate. I run upto 2 bar (2.04kg/cm2 to be exact) and I still see a 15-20bhp increase from water methanol injection. I believe alot of this is more due to the methanol than the cooling as I haven't been able to push over 230bhp with the water/methanol and diesel alone. If I up the diesel (by increasing injection pressure/timing) or if I increase the methanol% it makes no odds, I get no more and can lose bhp/lbft.
If you don't have an intercooler, the difference you see may be even larger than what I have seen but it will work on top of an intercooler.
I'm now try out adding a butane/propane mix, added through the intake. This is added at the filter in gas form (I tried liquid and the results were explosive!!!!
my poor car was bleeding oil) and the water methanol (20-30% methanol as I'm adding other fuels also) is injected after the intercoolers. I have had reading of 246bhp but I need more testing to be sure.