Re: 1996 zxi 1100 speed ?
I don't know about "set to read higher" - but the speedo pickup is just a little set of cups that catch water and spin as the ski moves forward. Not very accurate and those of us that ride and wrench on skis don't call them "dream-o-meters" for nothing.
as far as your ski is concerned, 48mph even per the speedo sounds low. I believe an 1100 zxi is capable of mid-50's (stock) and lower 60's with some mods done on it.
I think ride plate and intake affect handling more than speed, but they all kind of go together. To get more out of your zxi you can increase engine compression by either milling the head or installing an aftermarket head like an ADA, change the exhaust system (Coffman's is a good choice if you can find it), advance timing, and finally port the cylinders and/or cases. Replacing the impeller is a good idea, and if you can find a set of "Beach House" adjustable sponsons - they'll help with handling also. Do some internet searching for James Crone and Performance Engineering - he was a zxi guru for years and squeezed every bit of performance out of them.
be aware that the more you start modifying, the more chance you take that you'll be rebuilding or replacing that motor sooner.
If you really want to go fast - sell it and buy a more modern ski with a turbo or a supercharger. Some of those are capable of true speeds near 70 mph right out of the box.
and you should know that the zxi hull is not very stable at high speeds - it will throw you when you least expect it...which is why mine stays in the garage.