Glad to hear you are beginning to feel the balance, and even better to feel when things aren't in balance so you know you need to do some additional tuning and "tweaking".
Not sure how far (inches) you moved the mast foot, but in my experience all the way forward would probably not be the fastest or best position.
Can you now try moving it back a little at a time and see if there isn't a position that gives you the same control, but better speed due to a little shorter waterline length when fully planing?
It may be that due to the small fin size, you need the all the way forward position to get the fin and rig to balance, but then again, maybe not. Give it a try.
I used that same strategy last year in Bonaire where the max. fin size is about 33-34 cm due to the very shallow water, and it really did solve some serious spin out problems, but made the board feel very "sticky".
Yes, changing the type of harness you use can change the boom height and harness line length substantially. Do you have adjustable length harness lines? Try some and if possible, try some "on the fly" adjustable lines like the Sailworks "Quick Tune" lines http://www.sailworks.com/06/moreinfo...?Product_ID=82
so you can adjust the lines as you are sailing to see what differences having shorter or longer lines can do for your control and speed.
Getting the line length to feel "perfect" can help a great deal to "adjust" your stance, give you some indication when your boom height is high or low, things like that, that really do affect the overall balance of the rig to the board.
It seems you are still fighting with the chop a bit.
How big is this chop? Knee high....; waist high...; bigger?
Chop mostly affects the fore and aft (pitch) attitude of your board, so it may be that either your weed fin is also being adversely affected by rapid changes in the pitch trim of your board (causing your spin out) or you may be trying to compensate somehow when you see the chop, and your compensations are making a bad situation worse.
What I have learned about short wide boards (like your GO) is that we often try to control things (like pitch attitude) by moving our weight, changing our stance, changing pressure on the feet, and when we develop our skills a bit further we discover that all of our "compensations" are actually causing more problems than they solve.
Give this a try.
When you are sailing along at your fastest, just try to let the board "balance out". Don't try to steer it around the chop, but rather kinda let it pick its way through the chop.
You just keep a steady "power pressure" on the mast foot, steady foot pressure on the fin. Often you will discover that the board just seems to glide through or over the chop that you were compensating for.
Try to keep the pitch and roll attitudes of the board consistent and I think you may find the board settles down and the fin will stay "stuck" quite a bit better.
Hope this helps,