sorry this post is very late, but it might be of interest to Hampshire members :-
http://www.tritonmotorsport.com/showpage.php?id=index_cont
this garage is close to me, he'll rebuild the 2015 WRX STI to 400bhp for less than 5k, including Remap.................this includes a total bottom end rebuild, i.e new main bearings etc............... the top end will need new pistons/ con rods etc.
he said roughly what i said `` if you want 400hp dont bother buying a new car, but one with around 10-15k miles on it, because to be really safe the whole engine needs rebuilding.......................he was a nice bloke on the phone.
i quite like the 2012 WRX STI R340 and he said, ``it's the same as the 2015, it'll still need a rebuild``...........
this ``really safe engine query`` is sort of up to you....... you might be ok without an engine rebuild on a newish engine, you might not, there is no clear answer on this.