I think that the (mainly Pacer operated) Sheffield - Lincoln service would be more suited to upgrade to a 156 (to recognise the longer distance nature) than downgrading to a "heavy tram" / "tram train"
I agree that sprinters would be good for this route, however I think that it would depend what these heavy trams are like, to if they are an upgrade or a downgrade, if it is a lightweight body with bogies and suspension. Then if it was outfitted right, we could be looking at something on a par or possibly better than a sprinter for passanger comfort. My main worry would be life, are they specced to last 40 years? Like it seems most DMU's are expected to run for?