You can change that in two places:
1. gdb track file
2. rfm mod file
Unfortunatelly, gdb has higher priority than rfm so whatever speed limit you have set in rfm, rF will use what you have in gdb. The solution is, to delete speed limit from gdb file and release it as your league patch for that track.
I hope it will be sorted out in rF 2. For example, you can create championship profile and there define all rules for the whole season and also for particular events. All within that one profile.