Anything above a 3550 is tough to shift fast in a heavy car, I believe it's the synchros are just better suited to lighter cars and I would think a heavy car drags the tailshaft speed down quicker between shifts. 1-2 is the worst and it's made even worse by the huge RPM drop even inthe tighter ratio box. 2-3 and 3-4 I've never found bad to shift fast but I think the Mustang guys break 3rd gear a lot.

I have a Tremec in my Cobra Replica and I've just gotten used to having to shift it slower, I took the short throw out and put in the original backwards mustang shifter to remind me that I'm not driving a slick shift 833 or even a top loader.