The 155 is basically the smallest Seadoo engine where you can do some mods and it is at least worth it. That said, it is somewhat limited. Plus, it can get expensive fast.
Changing the grate can change how the ski handles as well as feed the impeller more water, (top loader for example). That said, an aggressive intake creates drag. So much so, that without some other item to improve things, you will normally lose speed.
Cold air intakes help. You can get a tune and that will help with everything.
Impeller, you need to be careful on this one. If you get a too aggressive impeller, you will get out of the hole VERY fast and have good low end performance. But,,, you will lose top end speed. This is true the other way to,,, Faster impeller normally means less performance on the bottom end.
