My '87 Elite 150 will do a little over 60 mph indicated, give or take a mile or two per hour, depending on wind speed/direction. I weight around 230-ish geared up. My nephew swears he gets a couple of mph faster driving it, but then he only weighs 160.
I went with the Dr. Pulley sliders, and they definitely improved acceleration. I forget the weight I used, but it seems to me they were only about a gram lighter than stock each. I believe they were 11 grams. Expect a little rattling noise out of them.
You mentioned in an earlier post that your Elite had trouble idling when cold. Clean your carb thoroughly. Change your final drive oil. Replace your plug and your air filter. Wouldn't hurt to change your intake rubber. They can crack, and that can lead to a very lean-running condition, as Windburn mentioned. Mine was cracked at 2800 miles. If your belt's worn, it'll affect top speed, but I don't believe they need to be replaced until at least 8-10k miles.