In my tests and all I've seen in magazines cutting down the prop will reduce thrust but increase speed. With a slippery airframe you may not need the thrust just the flying speed over the controls. Although the thrust may be greater with the larger prop the plane may feel mushy because of the slow speed. Most props efficency is best at a tip speed of 325-375 mph. Narrow tips will not provide the same thrust a wide blades but do usually spin faster.
Make sure that when you cut down a prop you shorten both blades PRECISELY the same and then balance the prop.