First, why are you using .015" lines, when you can use .012's and make the airplane happier? It will have to fly faster to make enough line tension with the .015" lines. Longer lines would be a good option. Smaller planes don't like flying at slow airspeeds. That's why everybody says bigger models fly better.
I never had a .15X, only earlier "steel fins" (Rockets). What I read about the .15X at the time was that they didn't like as large a prop as the steel finned Rocket. Admittedly, I have a poor opinion of MAS props, but I'd suggest trying an 8-3 APC, perhaps even with more nitro and not less. Even the 8-3 APC might be too much prop, so I'd look at their lists (not what's available at the LHS!!!!) and see if there's a 7.5 x 3 or something in that range. APC's might look scary, and they might not look like a Y&O, but don't be surprised if they work real well.
Remember that the more "power", the slower you can fly the plane. We're looking for thrust, not horsepower, and not really static thrust, but I must say that static thrust is a good indicator of something good about to happen. Then again, if your P-40 is from a genuine Sterling kit, it may be so heavy that it'll never be "happy" flying slower. I had one of the "Me-109's" when I was a kid, and it was a dog compared to a Jr. Flite Streak. Ah, those were the days!
Steve