When you say lots of revs to spare, do you mean its not revving high enough? Your engine should rev to about 5400rpm (i think from memory). So firstly you need to get the revs right. Putting a bigger prop on will make your top end quicker but also reduce your pick up, the engine will under rev and put extra strain on it as its working to hard and loose fuel economy. If its over revving you will loose top end but gain on pick up and again knacker your engine as its revving its tits off.