seriously for 3 or 4 bhp.. do you even think you'd notice it?.. an expensive placebo effect..
if you've done as much as you can power wise then the next logical thing to look at is weight reduction (that will give you a better bang for your buck) and make the most of your current power..
i.e. the seats weigh an absolute ton!, there are carbon and fibreglass options for the bonnet and rear boot, light weight alloys will reduce unsprung weight, ditch the spare wheel and carry tyre weld etc