It wouldn't be a bad idea extending 2tph Glossop-Piccadilly to Airport stopping at all stations replacing the DMU stopper, at least then EMUs would be used on the stopper to the Airport, rather than DMUs which can run non stop. It does depend on whether the paths are available though
Asside from the whole reason why we're building a cord to get TPE services around Manchester in the first place, it would need to cross all paths to get to the airport line, and going via Victoria from Ashburys to access the airport is a big time penalty and mpst users of the Glossop Line want to be in Piccadilly.
And there are no DMU stoppers any more since the Cleethorpes service got made fast and a Northern 323 covered the stopping path with cleethorpes flighted infront or behind it to get down there.
Btw. the time it would take to get round is about an extra 20mins to get round via the cord and victoria, and thats with very optimistic dwell times etc from Ashburys. 10mins of that being getting through Philips Park Jcns.
Personally I just don't think theres any need to get a service running from Glossop to Airport, and the existing EMU paths can be extended through Piccadilly onwards to Liverpool or Bolton, if we end up with a very major re-think of all the paths through 13/14/15/16 at Piccadilly.