I haven't heard of a problem with the XLT as far as bucking, but I'm not real familiar with this model. The bucking problem seems to be primarily with the Yamaha FZR/S models.
You may be able to improve cavitation with a pump seal kit, but in my experience the difference is barely noticeable unless you are actually experiencing cavitation.
On some models it is not recommended to seal the stock pump, since it was designed to run with some leakage. I'm not sure about the XLT, but I know that they do not recommend sealing some of the other models unless you also upgrade the intake grate. If you find a seal kit for the stock grate/pump shoe then you will be OK.
I'd say sealing the pump is optional, and not a requirement on any ski. You may notice a slight improvement in the ride by changing out the intake grate/pump shoe with an aftermarket like the R&D. If you do go with an aftermarket grate, then I definitely recommend adding the pump seal kit as well, or filling the spaces with resin. I put a R&D grate on my ski and did the seal kit at the same time. The biggest improvement for my ski was a better ride in rough water with the new grate. The improvement was noticeable, but slight.........if that makes any sense.
The areas you are sealing are the hollow spaces inside the intake grate/pump shoe. You have to remove the intake grate/pump shoe to do the seal. You can use the aftermarket kits, which are basically just rubber pieces that you glue into the hollow spaces with 3M 4200 or 5200, or you can fill them with resin. See this photo for an idea of what I'm talking about