I think a lot depends on the hull. What boat are you considering these for?
Formula 353 gains 3-4 m.p.h., 382 possibly 2 m.p.h. Have not really been able to percieve handling differences between boxed/non-boxed 382's. I think it makes a little more difference in 2-4 ft seas. Probably not huge difference in prop/r.p.m. Not sure I would go through the trouble installing these. Bigger bang for the buck spending the money upgrading engine h.p. Mine were factory installed.