I think fair points made chaps. Also, more power also means more fuel consumption. Given the cost of diesel in this country, increasing fuel consumption is probably exactly the wrong thing to do.
As it turns out, I'll probably not get the head ported - although improved volumetric efficiency is quite an easy way of improving the efficiency of an engine - plus the advantage of keeping the intake charge cooler... the advantages aren't all at high revs
I've seen a LR Freelander autocross car with an M57R tuned to 190bhp. Couldn't comment on its longevity though!
Probably better to keep it simple - exhaust and intake modifications, and possibly a larger intercooler.
That said - I thought this might be an interesting discussion point!