Unless you are one who likes to "scrunch" up on a stock, at 6' tall, 15 is not necessarily too long. A lot of things determine LOP, grip style being one of the major reasons.
Interesting observation oneounce...
I think, and correct me if I'm off on this... the stance used, relative to the target will have a substantial effect on preferred LOP as well.
When using a "combat" shotgun, being trained to be nearly square to the target, i.e. shoulders nearly perpendicular to the point of aim, weak foot slightly forward (is this the "scrunched" you refer to?), I use a relatively short LOP stock. An example would be a Benelli M2 with a Mesa Tactical Urbino stock (with PG)... LOP is 12.5".
Back when I first got into combat shotguns, I used a Savage 69 with a wood stock. I eventually ended up with a LOP of 13.3".
I believe as you stated, that an actual pistol grip, or lack there of, has a bearing on LOP as well.
I'm not much of a skeet shooter, but having done a bit of it several years ago, I recall being quite comfortable with an angled stance, i.e. shoulders roughly 45 degrees to the point of aim, weak foot well ahead of strong foot, using a gun with, what I believe had a 14.75" inch LOP. Perhaps the light skeet loads had a bearing on this as well.
I've never had a stock fit to me, nor have I held a custom fit stock with drop at heel, drop at comb, cast, pitch, thickness of the comb etc. etc... because I'm a lefty, and you just don't encounter that sort of thing often.