Slight correction. Unlike the 350, the ecu on 370 does allow and keep performance increases, as it was found on mine - the Stillen Exhaust had added some extra 18bhp over stock (as claimed) for me a year or so earlier. BUT, those carrying out the uprevs have found the 370's are set to run overly rich from the factory whilst the mapping can be adjusted to suit personal preferences. What I found, and I believe it is much the same for others, is that although power increases can be maximised with the uprev to suit -the process of leaning off the mixture means the mpg is not adversely affected and can even be improved....as I have found on the motorway trips
That said, the synchro rev works so well that together with the mods/uprev you just can't help yourself enjoying the combination so then the mpg goes back down But like anything to do with mpg (and if that really is a concern) it all comes back to how you use your right foot