Hellcat can still hold its own against T8 - remember even of you have to launch the occasional "premium" shell down range at a T8 tank it's no biggie, as long as you hit and pen you'll still make a profit on that shell (shooting and damaging a +2 opponent has better credit multiplier).
The main issue vs t8 is that they will have better view range than the tier 5,6,7 that you'd prefer to meet. Just change the way you play, assume that you'll always meet T8 and gear up for it - also learn the camo/bushwork mechanics and exploit them.
Look on the bright side it's not the IKV 65 Alt II which as great camo, like the Hellcat, great 90mm gun, like the Hellcat, great mobility, like the Hellcat but crappy gun traverse so that every time you move to re target you lose binos and camo net.