Mapping is a big part - you can build the best setup in the world but if the mapping is off, the engine life will be reduced or the chances of parts failing will be much higher.
Also, the longer a tuner spends mapping, the better it will be (or should be, obviously).
I think in your case Dave, if you know it's running rich then there would have been a chance of bore wash whilst running in and the mileage afterwards - but not saying that any of this is necessarily why it's failed. There's also a chance the parts could have failed, of course.
You mention about your intercooler - both Errol and I have had that one and without being rude, it's poo. At those kind of horses I bet your intake temps are up around 50 degrees plus. Which in turn might be why the graph (370hp) waves whereas lower down (300hp) it's nice and smooth. It could be the high intake temps that's caused the piston to fail. My advice on this, fit the biggest one you possibly can - with your blower you won't notice any extra lag. You want to get your intake temps as close to ambient as possible.
I've always found that finding and trusting tuners is difficult.
Unfortunately these sorts of cars tend to have a lot of teething problems - I'm still ironing out/improving some of mine from 3 years ago.
You'll get there I'm sure