What if my FPS is just slightly higher than the limit? Can I still use it if it were let’s say at 405?

turn your hop up all the way up and it will slow your fps down by about 15.

They won't allow you to use it if it's that high

No. Has to be 400 or won't be allowed. Saw a guy get turned away for 401 the other day

Yes there is a small window of that happening. But they might ask for no to dump a mag to lower it to 400