Shouldn't be an issue using T-Cut, after all its actually the lacquer you are working on, the paint underneath is fine. T-Cut Mettalic is less abrasive, but either will be fine if you don't go nuts. I wouldn't T-Cut my car, but i wouldn't hesitate using it on 15yo bike.
But how can you tell whether there is lacquer Paul? I thought it was only metallic and enamel paints that were lacquered over, and that T-Cut Metallic was designed for dealing with dull lacquer.
I think the people who said T-Cut was a no no were alarmed at the amount of paint on their cloth using T-Cut. Maybe they were just not being careful enough as you say, but they were certainly going deeper than any clear coat that was there.