170431 and 170432 were trial fitted with the higher rated 483hp MTU engine that was subsequently fitted to the class 172 units. Reportedly this was not a success on Scotrail express services, and the two units have since been retrofitted with the standard 422hp engines, so I think that the shortcomings are with the transmission, given that the 172s, with their mechanical transmission, are considerably nippier.
I remember these 2 units kicking about with the higher powered engines when I was a conductor - they took longer to rev up, but were certainly noticeably quicker. Unfortunately, I seem to remember hearing that one of the engines caught fire...?
Again, the Class 170s are just not designed for long distance passenger work - they aren't bad trains, as such, just more suited to commuter type services.
The 'turbocharging the 170' idea (to steal a quote from HiTrans, I think) has come & gone. In fact, didn't this form the basis of FirstGroups last franchise bid, that Abellio won? Scotrail have chosen HST as the next step. I would agree, however, that the Stadler FLIRT looks like an interesting idea for the future - more so than a Hitachi Class 800 type. Indeed, I would like to see something like this replacing Class 158s, never mind the HST!