For which version of FPSC?
To be honest, there are only a few mods since the release of FPSC v1.18 that were of actual usage:
WASP Mod
RPG Mod
The above were also available first for FPSC v1.18 and v1.19. [EDIT] Forgot about the new LewMod available, but I have not purchased yet. However, from the looks of the features and video, it looks like a great mod as well and very stable.
As far as commercial rights, you are better emailing the creators/developers directly for a direct answer so you are not mislead in any way.
As far as framerate, from what we have developing on our own, none of the mods had much of an effect. A lot had to do with how we planned our levels, when to spawn, how to utilize other techniques, etc. But everyone's system is different, techniques and methods are different. What works for me may not for you, etc.
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