Plugs probably not causing any problems but wont hurt to change. I think you are going the wrong way with the prop. You might need to go to a 24 to pick up rpms. You will loose rpm with a 28. You might be running a different gear ratio in your lower unit than the test boat. 28 seems rather big for a stock 496 mag.

Velocity ships the 260 with a 1.5:1 lower, exactly what I have on the boat now. If it had a 1.36:1 it would have had a smaller prop. First step is to go back to stock, which is a 28p Bravo 1.

Also running a different prop may allow me to trim it a bit differently, which may gain me some RPM.

i spun a 24 hydro 5200 on a stock 454 mag (365hp) in a 280 so i can promise you a 24 should be to small on a 2 ft shorter boat with 50 more hp

also a 24 hydro is more prop then a 24 bravo

I just picked up a used (almost new) Hydro Quad IV 26p for $250 + Shipping. I thought that was a pretty good deal. So hopefully once the weather warms up again I will give it a spin.

My friend has a stock 26 bravo and it runs 4800rpms. full of fuel.4900 rpms empty 68-70 mph.

my brothers 280 496 mag ho 29 hydro done by hulls 72 mph.

Per Steve Stepp 496 mag 375 hp =26 bravo
496 mag ho. 425 hp= 28 bravo.
hp500 efi = 30 bravo

it may be something as simple as a bad spark plug wire, that boat has been siting in a yard somewhere for a while and who knows who has done what to it, maybe an idiot boat surveyer pulled the plugs wires off like a dumb ass pulling by the wire and not by the boot and damaged the wire while doing a compression check , when i pulled mine off for the first time the part that connects to the plug pulled out of the boot and stayed on the plug, and they looked like brand new high dollar wires, my buddy bought a boat that was sitting for a while when we went out for the first time we discovered that two of the plug wires were on the wrong cylinders,

I am going to get a better prop on it and see how it runs fully trimed out. I just bought a hydro 26p. A buddy of mine has a 27p 4 blade that was cupped to a 25p. So between those two props I will see what I get. I don't think a plug wire is the issue as it runs strong all the way up to 4500-4600RPM. The computer diagnositic showed no faults, but did show an inaccuracy in my Tach, so who knows @ 4600RPM if a discrepency of 150-200 RPM would put the boat were it belongs. Not to mention I wasn't trimed out much before the 26p began to slip. That prop may be all screwed up for all I know. It is pretty knicked up....

There you go!! good luck.