To answer your questions, I believe its a combination of all those things, unfamiliar, poor fin and tuning. Ideally when you get a new toy, best to aim for moderate conditions, where you can tune your gear. Volume isnt a problem
Mark is right about the stock fins, not that great, and the small one should have been 34cm, if its chopped down, it will be worse than the original. get yourself a venom 34 for your 5.9 to 7m sails.
As the tail width of the hyper is significantly larger, you will need a higher boom (At least nipple height for you) & longer lines (26-30", adjust slightly backwards of where you would use for your carve, gets the rig more upright and away from you) to get the same sailing position as the carve. . A wider tail means you can also carry larger fins and sails. You can run a 8.6 comfortably on the hyper. Set your mast base on the centre position, downhaul your sail to max, control the power by adjusting your outhaul. Most likely if your using a 5.9 on your carve, odds are you could be using a 6.5+ on your hyper. (bigger tail requires more power, ie larger sail in same conditions)
Here is a guide as to what fin sail combos will work for you:
Personally I would be ringing its neck with the 2 larger sail sizes, before using the smaller ones. There is no reason why you cant do 31/32 knots off the wind with a 8.6 on it.