I don't own a 91 FS, but my hunch would be that those props might be just a tad too much. however I could be wrong. I've heard some people complain about slow spool-up time on a Saito 100 and a 16x4W prop, so it would probably be even a little slower on the OS. I would think the APC 15x4W might be a better choice, but someone here might be able to provide you with better info. If those props are a bit large, you could try running more nitro. Let us know what kind of results you get.