Putting an IR filter on an unconverted camera will definitely lengthen exposure times, but there is less difference in exposure times between a fully IR converted camera and one that is full spectrum with filters. I have a full spectrum Nikon J1 and use a Hoya R72 filter and exposure times are a fraction of what they would be if used the same filter on an unconverted camera.
I can happily use the J1 handheld and get decent (short exposure times), I don't get Motion Blur unless intentional The big advantage is that I can just select the filter that I want to use, rather than have a camera for every occasion. But for the ultimate Bob is right, have a converted camera for each desired wavelength.