IMO the GSX doesn't porpoise much,,, its a pretty good hull... not sure a grate would help that much, but i've never owned one long enough to think about a grate, they've always run very smooth, even in light chop.
handlebars ? UMI is available, but might come at a steep price (over $300)and IMO its not a dramatic performance improvement for the average rider, but aesthetically they are very pleasing.
handling... I've always felt the stock sponson's did a nice job, but ya, it still slides more than cuts i guess, so a set of beach house would be a consideration... just be careful, much easier to get tossed with AM sponson's, so i recommend getting used to them slowly, and don't try any hard turns in the 50's.. you'll go flying

I honestly don't recall seeing AM sponsons on a GSX but I really don't see many 98/99's out there, and when I do, they've all been stockers.
personally, if I was looking to "mod up" a 98/99 GSXL, i'd go solas impeller first,,,, improved holeshot, more "umph" coming out of turns. and see if you like what that does before going deeper into mods. I can't really recommend a pitch but somebody should chime in with their .02 on that subject.
if you not happy after that.. then proceed.