The Potter 15 was designed to carry lots of cargo in the cabin, this puts her on her desired sailing lines. To get the best performance out of your boat, you have to get the weight forward. Sit as far forward as possible, and move all your heavy gear up front. Also don't use a motor over 30 pounds, the extra weight will slow her down. I added a 15 pound lead slug to the bottom of the forepeak locker. Forward weight is one of the most important things you can do to make her sail better.
Sail trim and rig balance are also very important for the best performance. The P.15 is under-canvassed by most standards, which means she is underpowered. If half your power is trying to twist the boat around, and you have to hold the tiller over to keep her straight you are wasting half your power. You have to trim the boat so all the available power is pushing the boat forward. When sailing close-hauled trim in the jib so you have about 1.5 inches of belly, then bring her as close to the wind as she will go and trim in the mainsail until it is no longer backwinded by the jib. If she is trimmed right there will be very little weather helm in 12 knots of wind. If you take your hand off the tiller she should hold her course for a while and slowly turn into the wind. If there is too much weather helm you can rake the mast a little forward, or you can pull the keel a little.
The P.15 sails best at 10 degrees of heel.
When sailing in heavy seas don't try to point too close. Fall off a little to maintain some speed. You might have to sail a snake like course to make headway.
The ability to make smooth tacks is very important, so when coming about don't shove the tiller over hard. Slowly and gently works best and you can maintain good speed. Don't sheet in the jib until she has come through the eye of the wind. Let the wind fill the jib and help to put her on her new heading. Practice this over and over and watch your wake to see just how good you are doing. A slower boat can beat a faster boat just by making better tacks.
Downwind, a boom vang is essential on a P.15, and it also will help hold sail shape on a reach. Also use a whisker pole to hold the jib out so you can sail wing and wing. Sailing downwind it is extremely important to get the weight forward. If you can lift her transom out of the water she will really move.
I hope this helps you to get better performance out of your Potter 15.