Ain't no substitute for cubes. Cubic inches that is. If the budget stretches to it stick a 235 hp Lyc in the front. A snip at £12k plus the Major mod, plus the engine mount plus the new prop. It'll go 10-15kt faster, mostly due to your wallet being lighter!
I know it sounds silly but if you want more speed your better off buying an aeroplane that was designed to go faster. Try a Commanche or a Robin.