We have a mixed bag fleet of 701C's and D's here and really don't much notice the difference. C to D is a drop in replacement, and there is a mod kit to convert straight 700 ships (A models) to 701Ds. The 701C and 401C are pretty much the same engine and the D isn't really that big of a gain over either... hell its just a few pieces swapped out, like a sport tuning I'd say. The -D engine helps a bit at altitude but isn't the upgrade that the Hawk really needs, though the fact that one day D engines started showing up and just got dropped in the aircraft, and that all our C's are being converted to D's when they go in for rebuild, proves it was an extremely smart logistical choice; getting a little bit of performance without so much as a hiccup on the flight line.
Looking forward to an upcoming introduction of a CT7-8 derivative, somewhere in the 2500shp range, with the wide chord blades. The idea was to make a Hawk that doesn't start losing performance the second it breaks sea level and I think that's the right track to follow.
Mike
Edit: From what I understand the "400" series T700s are marinized, with some better corrosion protection inside and out.