Perhaps you know more marplerambler, but I do have to doubt journey times would be increased by so much as 15 minutes. A new Ashburys link like the one previously proposed would surely only add around 5 minutes, even factoring in an extra stop somewhere near those new houses in East Manchester.
Then remember that electric trams have much faster acceleration from standing than diesel Pacers which slowly rumble out of stations. This would gain time all the way along the route or at least perhaps negate the fact that a tram would (more sensibly) stop at every station rather than skip-stop like some of the trains.
And at Manchester itself, a tram-train to Bury for example would then continue on the regular tram line through Piccadilly Gardens and Market Street, meaning you can get off there and not have to walk or get the free bus from Piccadilly, probably saving at least 10-15 minutes itself and making travel into the city centre so much more convenient. Plus if you ever missed a train, you'd know the next was only 6 or 12 minutes away! I'd happily trade a few extra minutes on the journey for that alone.