Timing and valve adjustments for yami 25?

I have a 2000 25 Yamaha four stroke. Every year I decarb the motor, change the plugs, change the oil and filter, change the lower unit oil, change the fuel filter and water seperator filter.

It starts and runs fine, but it kinda feels like it is a little more sluggish and slower at top end than it used to be.

I'm thinking about taking it in and having the timing reset and a new belt, as well as having the valve clearance checked. Anything else I should consider?

Your maint. schedule has been stellar on that engine to date. Find out what the recommendation for the belt change is and follow that. In general those belts are amazingly rugged.
As far as valve adjustments go, have you started hearing a clicking noise? If so there may be good reason to have them checked but again, in general, I have seen very little wear in that area in engines I have had reason to go into.

No valve slap. Just figured I'd have it done while its in being that I'll probably not be taking it back in for a long time. I'll get a new thermostat too.

Valve adjustment makes a big difference, or more specifically, it did on mine (also an F25ESHY). You never hear them make a sound. Figured while I was winterizing it a year or so ago, might as well check them since I don't think it had ever been done. Both exhaust valves were in excess of .020" and both intakes were right around .005", which is typical (exhausts tend to loosen up and intakes tend to tighten due to normal wear). Adjusted all of them to I think .010. I think the spec is .006-.009 intake and .010-.013 exhaust. Since normal wear tends to tighten the intakes and loosen the exhausts, I set them on the ends of the factory specification, with the intakes being a "tight" .010. First thing I noticed was that it starts better, idles better, and a little more top end speed.

The timing belt is usually not a wear item. Since they came out in I think '97 with the F25, I don't believe I've ever seen a need to replace one. They were all tight, not really worn, none had skipped. There is no tension adjustment, either. When they're cold they might be a little loose but once up to operating temp, they're tight again.

The clearance check and adjustment is really easy. They even put the specification on the valve cover. Need 10mm socket and 10mm wrench and a thickness gauge. IIRC those are the only tools needed.