We had a 60plate FFRS that we took from a standard 300bhp to 480bhp. I have to admit, I felt we ruined it by turning it into a beast that could put you in a ditch if you weren't careful.
I understand your anticipated gains is not in the same league as we were but bear in mind that the standard parts may not be up to the job when your increase power this much. We need to upgrade clutches/gearbox, brakes/pads, suspension etc. A 'few mods' ended up costing in the region of £20k that obviously we didn't recoup. Never again.
We buy cars now that we feel don't need enhanced ...like our C63, Boxster GTS ......contemplating a Jeep SRT next... but that might be a bit extreme!
I have had my F56 JCW tuned to 292bhp, mine was 240 stock, it feels much better for the extra power, certainly doesnt ruin anything. More BHP are the people to go to, they have a Cooper s of their own and they write their own maps