Good question, I couldn't find anything that had an answer as to whether or not the TPC300/400 step settings (80/200/400) can be used. I don't have a TPC 300/400, only an old PowerMaster. Jon Z would be the one to provide an answer.
I'm using DCS, with my Base1-L hooked in so I can control my TMCC/Legacy engines. The DCS handheld gives me a choice of 32 or 128 speed steps. My Legacy GP9, 3rd Rail E7, and Railking RS3 with ERR Cruise Commander all move slowly at the 128 setting.
I can't imagine how 400 speed steps performs, the engine must really C-R-A-W-L at that speed