I use an FT158 in these combinations (I'm 80-85kg, 1m80) for back&forth and deeper "speed runs":
Naish Stealth 2004 9m6 with Drake Race 64 (stock) or Deb R13 S 70;
Naish Stealth 2004 8m2 or Retro 8m5 with Drake Race 64 (stock); and
Naish Redline 2006 7m6 with Drake Race 64 (stock) or Drake Race 56.
The mast base is usually in the rear quarter to third of the slot, or up to halfway if extra control is needed. Back straps outermost & rear; front, outermost & mid.
With the 9m6 the Deb R13 certainly feels as if it's dragging, and appears to give a lower top end - but, if it's marginal will give the extra lift to get on and stay on the plane. The heavier rig takes more effort (from the wind or technique) to get the board over it's bow wave. The big fin needs more pumping/ooching to get up too - then get the lee rail down and let the fin do the work.
I've read that "FTypes like 'big' fins", but with enough wind the stock 64 and 9m6 is fine - if you can try a 66 or 68, go for it.
At the other end of the scale with the 7m6, the stock 64 can get too big very fast as the wind/chop builds. The 56 does feel as though it's not getting the board high enough to only engage the rearmost planing section - I'd love to try a 60.
So far I've logged on gps the 7m6 + 56 at 27 knots, and the other combinations at around 25kn - so, in their wind bands they're not too slow.
If the wind is marginal for any of these combinations, raising the boom helps with leverage and trim - when overpowered dropping the boom some helps to get some "weight" into the front foot to keep the windward rail under control!
Good luck - Martin