RE: sunken mast track
Get the rig a low as you can at the tack (i.e. no extra extension and pull the downhaul until the tack pulley is practically "two-blocked" with the pulleys on the mast base).
With your 7.6 (or is it 7.4) rig, I'd suggest trying the smaller fin first, and if that doesn't get you going, then the larger fin.
Hypersonics get fast and lively when you run the smallest fin possible.
Start out with the mast foot at the back of the track and see if you can get the HS 111 to tailwalk a little. Then move the mast foot forward slightly until it still very loose, but controllable.
Hope this helps,