Paizo has items that grant +15 skill bonuses, official material, not custom items. Off the top of my head there is Stealth and Escape Artist "add on" armor abilities that go +5/10/15. They run a bit over 30k gp for the top end ones, so not terribly expensive either.
If combat "skill checks" are going to be a "thing" in PoW, they should probably be shifted up a tier or two from what the developers have now. The games PoW are going to be in are home games, not PFS. This often means a more relaxed and less strict adherence to rules and absolutely does increase the likelihood of custom items. Not every GM is going to be able to screen and scale all things as needed, and it is significantly easier to scale something up in power than it is to scale down. Not to mention the sour taste that it leaves when an book ends up seeming OP or broken due to some small thing like auto successes in combat because of skills being brought into play (and swift action recovery mechanics).
Any time a 3pp suggests a possible rules variant that could be pushed to the limits like that, I would always suggest erring on the side of caution. If something gets called broke, people will will be significantly more critical of everything made even if the rest of it is solid. Seriously, just look at psionics.... And the bum rap they still get. Tone it down for publishing, and put a side bar saying, "we used these assumptions for design purposes, if you don't allow +15 skill items do xyz to bring it back in line with things as intended." Cover your bases.
There are ways to get +15 to skill checks in the game, items that don't take up a slot and are cheap. When designing the rules for using skill checks in combat, the PoW designers had better figure that into the equation, because something like that will get used. Whatever numbers they are throwing around should have that +15 included as part of the figuring. It is cheap and effective, and an absolute must have for any character that would make use of those manuevers.
Suggesting banning skill items because it might end up broken in regards to a 3pp publication is just... So backwards it is silly, in an unamusing way.