Originally Posted by
BluesDawg
Shimano says you'll need the new FD, so that's the official answer. Real world answer will have to wait until someone tries the new crank with their existing FD. If there is enough adjustment available on your outer limit screw to move the derailleur out 2.5mm, I can't think of any reason it wouldn't work just fine.
Someone has to be making a clamp on adaptor for braze on ft derailleurs that pushes the derailleur out more. If not, I sure hope Problem Solvers is working on it.
Of course, it might not be too economical to buy an adaptor plus braze on derailleur instead of just the correct GRX derailleur, but if you already have a spare derailleur...