This is a really interesting thread and it would be a pity if it runs out now so here goes....
(I suspect that Horton has a few of these types of questions in reserve to keep us ski geeks interested)
The original question from Horton was how to move his centre of mass (COM) forward in order to help him achieve 39's.
It seems that while everyone is agreed that the balance of angle / com/ load / speed / acceleration etc is what separates the good from the bad and ugly, the advice to Horton has come round to his "stack" position rather than just focusing on COM.
I have taken a few screen shots of pros running 39 at roughly the same spot that Hortons snapshot is taken. What I see is that, while the Pros have what I would term an "indestructible" position (meaning that whatever else happens - increased pull from the boat, the wake etc - they will still be in control and gain speed and angle), Hortons position is on the limit. Any increased load from the boat or hit from the wake, looks like it would result in more "compression". It's unlikely to be much as he's really strong in this position (it's his default), but any "give" means that the power from the boat is being dispelled before it reaches the ski. As the line gets shorter this becomes less manageable and you can't afford the time to fix the compromises anyway.
I would tentatively suggest therefore that moving the COM forward is probably not the main issue (the pics of Andy and Rossi don't show any more forward lean than Hortons) but gaining a more "controlled" stack position that, as things get more on the edge, generates speed and cross course angle rather than load and compression.
Establishing that, a question I have about this (which probably applies to all levels) is how in the world do you approach changing a default stack position that has been refined over many years? Is it predictably possible?
We have skiers with us who have been skiing and competing for over 20yrs and, whilst always working on stuff, still have the basic compromises in their stack. It may of course be as Marcus says that they are simply not physically able to achieve the position required but I would suggest that if you can do it on land then it should be possible on the water.
My personal preference is to get away from the course and video every set, making and recording all the changes you try until you start to see a difference. I think you also need a reference point of video/pictures of a skier you think you want to ski like. As has been said before what you feel and what's actually happening are generally very different.
Where I may now overstep my pay grade is suggesting that from Hortons photo his goal may be to find a stack position that allows him to lift his right shoulder whilst keeping the right arm locked and loaded. This would have the effect of a more upright "taller on the ski" "indestructible" position like Andy and Rossi. Clearly would depend on the turn, hook up, tight line yadayadayada.......
Hope this thread continues.......:)