I replaced the IRFP3710 MOSFET (T3 on the schematic) with an IRLIZ24 MOSFET. This bought about 2 more volts of headroom for driving T3. I took it to 1550 amps and did not see any aberrant behavior. There is more headroom left, but I only checked it out to 1550 since the highest number I heard for this experiment was somewhere around there. If we need to go higher, just let me know.