I'd rather throw up at the opportunity of seeing 75mph Turbostars on the Cardiff Portsmouth route ... there is a high linespeed between Filton and Cardiff you know...
172s are 100 mph.
172s will struggle to get to 100mph - they're designed to be highly geared to allow for high acceleration at the low end of the spectrum, optimum acceleration will probably be up to about 55mph... then a sloooooooooooooooooooooooooow increase to 100mph. I know where I'm coming from on this one so I wouldn't try argue it!
170s would be more suitable - shame the production line is currently full with 172s. Diesel 444 is still the best bet.