Just ECU reprogram nothing elese . What did you gain in MPH. And who did it?

Depends on set up and boat. Are u on rev limiter? Spinning them 5600 was no problem
Mark at Precision Marine

I was on rev limiter and turned it up 100 rpms, gained solid mph, 1.5 sometimes. Timing curve is stupid when you look at it, goes up from 28 to 36 degrees timing in 100 rpms at 4900. I tuned one on water for a 27 fountain and it had afrs in the high 10s low 11s at light throttle, when we were done hes getting 20% better mileage cruosing BUT i wouldnt start turning fuel down unless you were tuning on water in your boat

You won't gain much, if any, in pure top end speed, although 1-2 mph is possible. You can gain a ton in economy as artic mentioned. They are usually quite rich. There is a little power to be gained by increasing the limiter and adding a little timing, but you will have to run at least 89 octane. We have what I think it a pretty good tune for the 500's. A larger monoblade throttle body along with the tune will help as well.
Eddie

My fountain customer called me after tuning his motor we built and wondered why he is only using 32 gallons vs 40 to make same trip he was previously doing, pretty happy with economy gain and we still kept tune fuel safe..

Where the early 500 efis a little down on hp. Compared to the later model, s. I have the early ones without the mechanical fuel pumps.

I allways bump up the base timing 2 deg. I could be my imagination but it doesnt seem to soot up the back as bad.

If you already bumped the timing from 8 to 10 base which puts your timing at 37.9 degrees total from 5000 up (38 for discussion) then unless your on rev limiter I highly doubt you will see any top end gain from any further tuning