Hi Giles, u are almost the same size as me-on freeride or slalom i think that stance is fine. You can maybe get a bit more speed by going back on the back strap. But I would leave it as is.
Grips-the back strap further back will give more turning, but is then also a pretty wide stance. Further fwd on front strap may add a bit of planing, but then also very wide stance.
If it was me, i would leave them exactly as you have them for best allround performance, but feel free to play around a bit!👍