Hopefully Bob will chime in here but I believe this engine also got the Whipple repro....
Dave

I upgraded it from the 375HP 496. We installed the HO cam and sent the PCM to Whipple for his stage 1 program, bringing it to an HO and raising the rev limit to 5400. I also removed the turbulators and did Raylar's porting trick on the exhaust. We tested with 2 people, and about 40 gallons of fuel. I believe there was a little more speed left in it. The trim switch wasn't working on the throttle so I had to trim it on the dash and I don't think I ever got it trimmed up to the perfect spot. The Rage needs a lot of trim. I've always had good slip numbers with the Rage. If you are getting significantly higher numbers, something is not right. Maybe your prop doesn't have enough cup in it? This testing is with a brand new stock 26 Bravo 1 prop. He has another 26 that was damaged and repaired and it doesn't perform nearly as well and slips a lot more.

Here is an old comparison on my boat (BOAT #2) versus the 2003 Power Boat Mag test of the same open bow Rage with stock 496HO (BOAT #1) . You may have seen this before. This shows the slip I get with the stock 26P Bravo and the labbed 28P Bravo1 I am running now. These numbers are without the open bow cover. With the cover, just the driver, full gas tanks and perfect water (6" chop) and perfect atmospherics (better than sea level oxygen content) I was able to get 75mph at 5000 - 5100 rpm with the same labbed 28P Bravo (remarked by the prop shop to 27P).