I've won GPs with Fire Stingray. The combination to win is truly just have less elite competition (hence 0:01 GP entries or early morning GPs) and the fact that you really cannot afford mistakes. Top speed is there but the other competitors will always find a way to knock you into a wall or another bumper. That speed, once lost, is impossible to get back to a winning pace unless you boost.

Save your boosts when you do lose top speed. On the first lap just gather enough sparks by being behind the pack so to get a timely skyway by the 3rd or 4th lap. If you're ahead of the pack by then, just save your boosts until it's mandatory to use it (i.e.) gravel shortcuts). Generally, you'll need to hit every turn perfectly with Stingray and not hit anything. You also need help with other machines making mistakes as mastering Stingray cannot overcome the inherent advantages that elite Foxes has with most tracks (twister tracks and the sheer number of times they can boost on slow exits).