You should be able to sort that ECU overboost light out by adjusting the bleed valve that you have.
Seein as you have a Calibra, you should go for the intercooler option from
Wallace Performance. Biggest intercooler you can fit.... probably around the biggest fecking intercooler any car can fit
Water injection aint used much on any normal cars at the moment but its getting popular. Its not worth having unless you are running boost of around 20-25psi as an intercooler etc will do the rest or unless you are tight for space. You aint, so I would go for the 8.1 litre intercooler.