What are you running now?

I have a 51 rear on my 06 with a 13 right now. I've also had a 49 and a 45.
I don't think I'll be able to get a 14 or 15 to work in combination with the chain length as is. With 13/51 and a new chain, the snail adjuster was at around 9 (max) and there was more slack than I wanted, and cut the chain. Now the snail adjuster is around 2, maybe 3 with the 13 cs, and going to a 14 or 15 may just be too tight. I like changing out front sprockets for different rides.

My suggestion is also consider a 14/48 or 14/49. Then if you don't like it, you can switch to a larger or smaller countersprocket.

 

I moved from the stock 14/42 to 13/42 and love the difference. I rarely get above 45, and I feel like I've found the sweet spot of the bike for the local driving/back roads that I've ridden so far. After I get some trail time I may consider moving to a larger rear sprocket. Aside from chain length and clearance issues, I can't imagine any other issues that might arise. Keeping the chain clean, lubed, and tensioned should avoid any binding issues.

I have a 50T rear sprocket in the garage. I'm toying with the idea of 13/50, just to see how crazy it can be. Who knows, maybe I'll turn rock-crawler. I'm sure it would be useless on any road, no matter how narrow and windy.

 

It won't be useless, you just might be fairly tapped out at 60, but otherwise you'll have some serious 4-6th gear back road fun in the powerband on your way to the trail.
Straight aways are for relaxing at a casual gallop, but the twisties are where it gets fun and the KLX shines, as well as off-road.