Bob,
We have sailed only in about 25kts so far with the setup and have not had any issues other than the top block does not like to rotate on tacks and as you said you have to be diligent about keeping the blocks in column and pointing aft of the traveller.
However to get around the speed issue of dumping the fine tune I cut the sheet down to the correct length and added the ball at the end. Now all you have to do is quickly uncleat the finetune and let go of it and it zips to the bitter end and once it has been uncleated and eased a bit the load comes off and then you can uncleat the main. It also gets the line out of the cockpit. In light air we do not use the fine tune so it does not add to the rats nest since it is cut down.
I built this setup from the info you gave me and it has made an amazing differnce in our abilility to trim the main.
Thanks,
DB