Perhaps what your dealer is suggesting applies to getting the sail tuned for it's mid range.
Sure, you can "overdo" the downhaul, but it's hard to do so without breaking the downhaul line or something, and I've never seen extra downhaul affect the camber rotation unless you don't add the requisite amount of outhaul to match the amount of downhaul.
Downhaul/outhaul need to be adjusted together so you maintain some balance in the rig.
Lot's of downhaul and no outhaul is most likely just as bad as minimal downhaul and way too much outhaul.
Anyone who says harness lines need to be long, on a wide board like the F-Type just doesn't get what we are trying to achieve here.
For sailing upwind, and reaching, the lines need to be adjusted so your entire body weight in on the harness and there is almost no pull on your arms.
How long is this..........? it's entirely dependent on your physical size, the length of your arrms, the width of your board.
You want to be in the "figure 7" stance, with your arms fully extended (maybe even with your shoulders rolled forward some) so your upper body is as far from the rig as it can go and you still can get your hands on the boom.
BUT, the lines may be quite short as the harness and lines need to "lift your butt" to get all your weight on the rig. The harness lines are not "lifting your butt" up off the board, but rather supporting your butt in the "figure 7" stance with all your weight on the harness and your arms fully extended.
To sail downwind, really deep, on a formula board, you do adjust your harness lines quite long, but you also bag the sail out by releasing most of/all the outhaul to make the sail efficient that far off the wind.
For recreationsl sailing, you will probably never sail that far off the wind (this is like as near to straight downwind, with the rig fully sheeted in and raked back with the board sailing on apparent windspeed exclusively).
As soon as the formula racers round the leeward mark, they shorten their lines alot, and add outhaul to give the sail it's upwind profile to get max. performance upwind.
Again, harness line length is governed by:
Your arm length...
Your stance (which we hope is the "figure 7")
The type of harness you are using.....
to some degree the kind of board and kind of sailing you are doing.
Small boards/wavesailing/waist or chest harness/ very vertical and "over the board"
stance= very short lines!
Free sailing/mid size/width boards; waist or seat harness/more "laid out to the side" stance = lines adjusted to your arm length.
Formula racing, very wide boards, seat harness, very "out off the board" stance, seat harnesses, adjustable length harness lines and adjustable outhauls= very short for upwind, longer for reaching (formula racers almost never are on a reach) and quite long for downwind with bagged out rig, and a change of stance to face almost forward.
So, harness line length is quite different due to alot of factors, and mostly people tend to have them a little on the "too long" and too widely spread out side of well adjusted and balanced lines.
Hope this helps,