Hi John
A couple more thoughts from downunder
The fact that it comes down more easily than up probably does eliminate the boltrope size as an issue.
Agree with the luff curve Vs prebend issue - very common in the Tornado and Paper Tiger boats I sail a lot. With the Paper Tiger we can pull some prebend on because the boat has a set of adjustable lowers (works great when sailing too!). Not so easy with the 'T'.
Check the sheaves and make sure the mast always faces the sail when you're raising. Also check for extra reinforcing (i.e. thickness) around the boltrope at the head (and the tack) - although I wouldn't expect that to be a problem with a Goodall.
We had a problem a while back when a batten end popped through the plastic batten socket screwed onto the luff - the batten end was just thin enough to enter the track but you couldn't see it. That meant a mysterious and very heavy halyard load but easy to pinpoint!
Finally as you mentioned, I have seen a lot of people regularly spray silicon up their boltrope. This does help a lot but you do end up with a very ugly black-stained boltrope - a last resort only!
cheers