going from a 3600 to 4500-5000 stall

Converters are almost always tighter than advertised. I have a 5k I can footbrake to 3500. Sounds like you are in the ballpark though, more like a 3000-3200 which is pretty spot on for a street driven setup.

Just curious what is the weather like were you are racing...? Temperature / % humidity etc...?