I have installed the Whipple Chargers on my 525EFIs. We did a mild package and got 768HP with 740ft/lbs of torque. We also changed the pistons (to flat tops) valves (Inconel) and rebuilt the engines. They are very reliable, great power and acceleration for a pleasure boat and beleive it or not the fuel consumption is almost the same as when they were 525's.
Thus, I am very satisfied. However, I beleive their are two tricks to know about converting the engines.
1) change your pistons to Flat Tops and make it a real Blower Engine.
2) make sure you have the correct tools (computer software approx $500) for tuning the engines and setting the timing, TPS and etc.