The converter can be the star or the dunce in a combo like that. Stall speed is just one variable. If you are set up to try different converters then I'd keep your existing one in the car at first and try it. If it doesn't work as well as you want then you have a data point to use when talking to vendors. I highly recommend a simple data logger that can record driveshaft speed and engine rpm when sorting out torque converters on a street/strip type of car. A race car is a simple environment compared to a street car. With a street car you want to keep converter slip under control when driving on the freeway but race cars don't worry about that kind of stuff.