I just got to Neptune. Before that, my highest was Jupiter. I finally figured out what I was doing wrong.
The trick to starsliding is to not shoot directly straight up. It's the same as tailsliding. When you try to tailslide going straight up, you just sit there and then splash and lose your combo. When you try to starslide straight up, you don't go anywhere. If you want a speedy starslide like the kind you need to get, you need to jump up at an angle.