decat - Just makes the car noisier and free's up a couple of HP
remap - Can't be done with out some form of piggy back or standalone ecu. Even then its a waste of time and money
fit turbo - Best option really but not cheap to do it right and you will be limited to the amount of boost you can make. The gearbox won't stand up to the increase of torque for very long unless you drive it like its on life support which defies the whole point of extra HP.
the cost and expected increase in BHP - Anywhere around 2k - 5-6k+ depending on parts used, who builds it and how far you push it. Again engine will be limited on boost so at a guess 350-400hp at most.