Yes, it looks like there could be an issue. That mod only modifies the vanilla locomotive and "OS Speed Train" locomotive. If it loads after MUTC, then MUTC will create the MU version of the vanilla loco with base stats rather than modified stats. I could add a hidden dependency to make sure it loads first, and MUTC will create the MU version with the modified stats.
Honestly, Bigger Slower Trains would be way cooler if it looped through every locomotive during data-final-updates, then it would be compatible with every mod regardless. I posted a comment with suggested code to do this.
