The stock set for your JP 8-10 are pretty meaty flexy 2 +1 fin set up , if you are still sliding it means your back foot is not on the tail pad or kick. When you are surfing look down and see where your back foot is on a cutback or top turn , I bet it's not on the kick. The new 8-10 JP has been refined and designed to be turned off the rail. Rail turns give that really smooth powerful arc on your cutback and takes the load off your fins. But the larger rear stock glass fin and plastic sides will flex under load and still let go .
That's why I like to run a true thruster set ups on my boards with medium raked & stiffer fins. For you Paul I think for you my full carbon FCS V8 front's with a matched V8 USA slot box rear is the go. I feel it's really important to match your fins , you would never drive your car with different sized wheels or tyres. This is a shot of my JP 8-6 with a V8 thruster set up which I was riding in our big swell a couple of weeks ago and would really suit your 8-10.
Also take note on the marks on my rear fin. It's really important to tune take rear fin. Surf with a stubby screw driver in your pocket. Just loosen the screw a bit (don't undo do it completely) and bump the fin forwards or backwards. Try it the difference is amazing. I can do these V8 thrusters delivered in NZ or Aus for $120.
This is the video of the big swell we had at Snapper. (sorry no Sups)