I'm running 15x6s on my 91s on a H9 P47 Thunderbolt, and a Sukhoi. They're heavier than my Skybolt. The engines haven't shown any problems with the 100degree weather we've had recently. I also happen to run a 15x6 three blade on the P47 now, and even that "not as efficient, way too much prop" hasn't heated the 91 up any more than before. Yeah, a 15x6 THREE blade. And it turns a bit more rpm than most of the two bladers I've tested with it. Yeah, not supposed to happen, but you know what...... there are lots of things the experts know that doesn't happen the way they say.

As for your maybe being just too lean. Good possibility.

Until you hear the word on heat temps, try clicking a couple clicks rich next flight. And if you don't see exhaust for sure, the next flight, click a couple more rich. You won't lose 5% of the power available, and that'll still leave way more than needed for that engine/airplane combo. And if you were flying too lean, you'll actually be getting MORE power. It won't be as high tech, but it will answer your question about maybe being too lean. Lot's of ways to do things.

As for you running the engine too hard. Not probably. The airplane is lighter than anything the 91 was designed for. Lots lighter.