| Not too sure on the first question, but I can try to help on the second.
More BHP usually means less MPG, but theres a few members on here running reduced pulleys, iridium spark plugs and an intercooler and getting better MPG with an increase in power too. Smaller pulley, plugs and new belt is around £350-£400, with a GRS intercooler costing around £400 brand new | 
