hi jordo,
these days i am completing 90% of my starboard side tacks on a 95lt fish. (i'm 96kg so a definate sinker for me) but i a not planing through them
a couple of things i do.
firstly. cheat!!!!

when learning them i did them in shallow water. when the nose sinks it hits the bottom and this gives you some extra time.
i find taking a big step around the mast helps. when placing your foot on the other side make sure it is just behind the mast. so you are not weighting the front of the board. (this is really important)
another thing is to jump around real quick. faster the better.
i still sink the nose all the time and end up down to my front knee in water. this is mainly becasue i didn;t place my foot far enough behind the mast when i took the first step.
if this happens all is not lost. when you are on the new tack and the nose has submerged. push down hard with your back foot and take the weight off your front foot. the nose will re-surface really quickly.
sometimes you need to tilt the rig towards the nose of the board to get things turning downwind. by moving your hands further back along the boom. (mainly if you don't turn far enough into the wind when initiating the move.