As in #4, extra horses rarely makes the car go any faster, it just loads up other parts of the drive train which might not be up to it and simply changing one component can highlight deficiencies elsewhere but if it’s serious top trumps down the pub you’re after and you have deep pockets, go ahead, spend and enjoy! If you just want the car to go faster there are much cheaper, more effective ways of going about it, cheapest of all is to learn to drive it properly, first stop is Carroll Smith’s “Drive to win”, borrow my copy, it won’t cost you a bean. Next sort out the tyre pressures, the difference is huge. If you’re ready to start spending, a decent chassis set up to maximise what you’ve already got is next, and if you can afford it, put some decent dampers on it, the best ones you can afford or get the existing ones serviced. For racing or serious track day amusement beef up the brakes or even just optimise the pads you’re using. Don’t know what diff you’re using but going down to a 3.9 gives the car extra zip. For me, only when all the above is ticked would I even consider touching the engine.