what can you do to a 15hp outboard to squeeze a little more power out of it?

Sell it, and buy a 25hp

I don't think there is much you can do. I know you can upgrade a 9.9 to a 15. but upgrading a 15?

I have a 1989 2 stroke 15 merc (with 9.9 stickers) not an up graded 9.9, it was a 15 to start with. I did three things that help out a lot to get some more speed out of the boat, but nothing gave it more HP. This is on a 14' 10" Starcraft V hull. 15 HP short shaft tiller Merc. 6 gal gas tank, 1 battery, and bow TM. The speed I was getting was 9-10 mph now after all this I get 14-15mph. I know 5mph doesn't sound like much, but at those small speeds and hp its was 50% more speed for my boat.

1. put a set of dole finns on it. The boat would not get the bow down and could not plane. with the dolefinns it planes right out fast.

2. I put a mini jacker on it (small jack plate) this  added about 1-2mph

3. The biggest thing was I move weight around in the boat. This is trail and error. You have to try stuff in different places and see what it does.

Ex. the battery for the TM sits in the bow. Now I couldn't get the bow down to start with. when I installed the dolefinns the bow came down; However it was digging now, so I moved the battery to the back of the boat. This help greatly.

Also I don't have a  livewell, so I use a cooler as a live well, that can can be put in the boat and taken out. Depending on if I am by myself of have a buddy in the boat. the live well will sit in different spots

i wish i could put a 25 on it, but i bought this boat specifically to fish 10hp lakes

You don't want to get too rowdy with a 15 HP in those 10HP lakes.  Those Fish and Wildlife guys aren't dumb.  If you get going too good and start passing all the 10HP's around you, they'll notice.  In addition, they'll stop you and it won't be the stickers on your motor cover they'll want to look at.  It'll be the Model and serial number labels on the motor and they'll know what they mean.

Just so you're aware,

Dave