There is no doubt you notice a difference, but why does it necessarily mean a performance gain?
I also notice the difference between a stiffer and more flexible frame going uphill. Not so much in a field sprint because I don't field sprint. But what I have noticed the difference only requires a different riding style and I prefer the rhythm of the more flexible bike.
I consider it a performance gain because it has added a new dimension to my riding.
I was always more of a spin up the hill guy, and still am most of the time. But, having the out of the saddle option to get up hills has changed things for my riding. Out of the saddle climbing on the 760, I felt like I was dumping a lot of energy into the tires. Having swapped the tires from the new rig and the 760 as an experiment I realized that wasn't it. Something sort of feels like a dampening or absorbing effect on the trek that isn't there on the custom. I'm not even sure it's all due to the frame, might be the carbon fork vs the steel one or other factors.
Now that I have gotten more comfortable out of the saddle, I've been working on proper balance that allows getting up the hill out of the saddle without anything close to hammering (useful on the longer rides as it seems to use different muscles). I also have been trying to remain in balance when hitting it harder (but when I tire it is much more difficult). So these are two options I didn't use much before and one of those is available below AT. Spending a lot of time on indoor spin bikes, I've tried to focus on form (partly to make the indoor riding less dull). After having focused on it, I admit I am also more efficient at it on the 760 and do it on that bike more often. But it doesn't yield the same result.
The above said, I also value the "century ride" more than "stiffness". There are many stiffer bikes than what I ride that I wouldn't be happy with. Having a smooth enough ride with enough stiffness to be satisfactory is probably a different thing for everyone. When I say performance it's my personal take on it, difficult enough to describe and pretty much impossible for me to quantify.