Probably somebody like Tam would be the best to answer this - he's probably tried it on more than one occasion. Myself, I don't see why there would be an issue with a larger pipe. After all, the static thrust is greatest when it mounted externally without any pipe, right? I don't think that convergence is that much of an issue with turbine pipes because the exhaust velocity is so high to begin with - its more of an issue with DF's where careful balancing of static vs. dynamic thrust is necessary because of the low velocity and mass outputs...

Bob