It's likely on the soft rev limiter. If it goes to the exact same RPM with both props you would think it has to be electronic. Are you getting an alarm?

...

No alarm.

Possibly, depends on how much power you are making where. I'm curious about the 1 mile an hour difference between the 26 and 28.

28 to 26 pitch should be about a 400rpm difference, but only going 1mph faster with the 28 at the same rpm also makes no sense.
It sounds to me like one of the props has prob been modified or the 28 is way under spec on pitch.

You really need to run the boat up to 5170rpm with a scanner hooked up. The scanner would verify your tach is displaying the correct rpm and will also tell you if the rpm limiter is triggering.

Agree with Griff, I'd make sure your tach is correct when you're trying to be this precise. Generally my rhinda scanner shows rpms slightly different than my dash gauge.

I run a 496HO Raylar 800 package with the Whipple. The rev limiter is set at 5,700 but with the various props I have I've only seen 5,550 with it.

Bad Influence,

I also call into question your speed at that RPM. I'm running a PQ 280 with a 496HO, M1 ProCharger and stage 2 Whipple tune, turning a Bravo 28P prop, and we get a solid 77GPS, 78 on a good day, at ~5100 RPM. Granted, we have a single stepped hull and you don't, but this, in conjunction with your getting the same speed out of a 26P and a 28P, tells me there's something off with your prop. Look for stamped numbers on the prop hub, usually on the rear face.

Thanks. Brad.