No need to go to 4,200 rpm, no power or torque there to speak of.
When you uprate the engine performance you shorten the service parameters to accomodate that, 18000 miles is seen by FIAT as OK on the original cars spec, you changed the spec so you need to shorten the service intervals, even if its just a filter and oil change, a good look over the hoses and coolant levels, and a check of the brakes, a short service if you like.
when i had my stilo remapped i use to change the oil/filters, plugs and clean/re-lube my induction kit aswell as checks every 6k.
i also think that part of the reason from reading many threads, especially from the input from the guy that races his GP in oz is when the manifold gets hot warping the turbo like rx7's have that problem! so your 1 or 2 140mph+ stints could have been the killer... and the 2nd just looks unlucky! personally i would get the ecu reset as if you take it back again with a dead turbo they are going to investigate more.