First up TR I'm not one of those
those super fast guys but will throw in my 2c anyway, also there are some agreed points above and some different opinions so in the end it'll boil down to you and what works and this means do it do it and do it more

I sail 99% rough open ocean which is different to river dam chop etc, I find river dam chop harder. My slalom board is 58 wide, I'm not a lover of wide boards. I find short fins don't work in rough water because I end up plowing through the chop, the same fin I used on the butter smooth water at Burrum and loved was a pain in the ocean on the same rig. My harness lines are 28's but I want to try 30's, I use a seat harness. I don't push off my back leg, I use my front but also bending as needed going over rolling chop swell as needed. I also don't belly my sail out like the speed guys, I find I have more control that way.
Keep your weight on your lines as this drives the board forward, do not sheet out, if it gets too hairy turn upwind and wash some speed off. Have total trust in your board, as Snags said let your board do it's own thing, if it gets ugly but doesn't catch a rail it will always come straight again. Don't look where the board is going, look way ahead and pick your lines. Fins are important, spend the money but get the ones that work in chop, I use Venoms, very happy with them (but have a mate who hates them), I was also very happy with my Falcon F1 before I changed boards and stayed with the Kiwi connection. There is nothing wrong with freeride boards & sails but chuck the fin that comes with them if you want to go fast.
Disclaimer, this is just what works for little old me and is no way universal

I remember reading a Lockwood post where he did a 36 off a Perth beach on a speed board, he can no doubt can or has gone faster, now that's a
super fast guy 
Here's a video from last year, cheers Uncle Bob.