It currently takes a 323 (EMUs that are generally seen as fairly fast accelerators) around half an hour to do the dozen miles from Piccadilly to Dinting, so I'm not convinced that there are going to be a lot of fast paths getting out of Manchester, given how busy the line is..
The problem with arguments for a modern day Rio on here are that they seem to contradict each other. Some people want a fast service to provide a competitive alternative to the Euston - Piccadilly service - fair enough (although HS2 will blow that market open in a few years time). But others are trying to provide a direct Manchester/ Stockport link to as many places as possible (Loughborough, Bedford, Luton), which doesn't need to be an ex-London service. Plus then there's the "let's use this as an excuse to re-open the Bakewell line" brigade

And the problem that geography means that one train can't easily serve Leicester, Nottingham *and* Derby on it's way to Manchester.
Plus, given the finite paths out of St Pancras, you're looking at a future timetable with four services an hour to Leicester, so does a proposed Manchester service replace one of these?
Do Sheffield/ Nottingham go down to just one train per hour (and potentially have to pick up stops at Market Harborough/ Kettering etc, slowing them down further) so that Manchester can have an alternative to the thirty-ish carriages that the current VT service provides?
If you want to fixate on "Leicester needs a direct hourly service to Manchester" (given that it's about a hundred miles away, so of limited use for daily commuters etc, whilst places like Wakefield/ Birkenhead/ Rotherham - which are in the potential commuter belt - have no service) then the best answer may be the two carriages of the Liverpool - Norwich service that detach at Nottingham - e.g. running them through to Leicester in place of the current "Ivanhoe" service. That wouldn't require any additional paths through the Hope Valley or over the Stockport Viaduct (since those carriages are attached to the Norwich carriages on that bit).