Sliding goose necks are and were never used to control the luff. The rules do not allow us to move the boom below the black band. Besides, why would anyone want a tight luff? Don’t you love those “speed” wrinkles? The only useful application of a moving goose neck is if the sail gets old and the luff rope shrinks. I suppose you could move the goose neck up as a temporary fix. But what you should do is fix the problem, the luff bolt rope.
The big problem with sliding goose necks is they never stay where you want them unless you rivet them in place. There is no good reason for having a sliding goose neck IMHO. And that is why modern rigs have a fixed goose neck. And that has nothing to do with having or not having a Cunningham rigged. Only some tent users complain, but I have shown in my previous post there are good, if not better alternatives to a sliding goose neck.
BTW a Cunningham does not control the boom, only the luff. As such it is a different mechanism as a downhaul/sliding goose neck.
Edit: leech changed to luff.