Based on the original post, it looks like RPM is down 100 from last fall and MPH is down 5-7ish. Here's a couple of random thoughts in random order from someone who boats up at 6200'. 100 rpm difference isn't really all that much much of a difference, I'd focus more on the speed difference. Although you're at the same lake, several things like wind, number of people in the boat, amount of fuel (essentially the weight in it) can affect top speed. A full tank of fuel of 30 gallons is just shy of 200lbs, or the difference of a little more than one adult. For example, on Lake Tahoe, I've literally held a steady course and RPM and I've seen my speed fluctuate by as much as 5MPH simply because the wind and resulting chop picked up.
As for fuel, if you left your tank mostly empty over the winter, there was a lot of exposed space for condensation to happen in the tank and the result is you could have some water in the tank. I'd recommend getting some Marine Sea-Bil and putting that in the tank now. Once you burn off about 1/2 a tank, refill and add the appropriate amount of Sea-Bil. Do that for the next few tanks and eventually the good fuel should overcome the bad fuel in the tank. Poor fuel could also be affecting the power it's developing as well.
To me, your plugs looked fine. I'd put them back in and then run it and see how it goes. There's a lot of debate about when to change plugs and some on here do it based on engine hours while others do it annually. I put about 65-80 hours on my boat per year so I'm in the annual camp. Although this may not necessarily be a requirement, in seven seasons, I haven't had down-time on the water because of bad plugs. I figure for the cost of a set of plugs, its cheap insurance to not have to spend time on the lake changing the plugs. I do carry spares...just in case though.
I'd give those few things a try and see if that helps her out and do some more testing, before going back to the dealer.
