The J-mo is a medium stiff ski for sure. But it's a perfect bridge between a park ski and an all mountain ski, so if you charge on it like an animal you certainly will notice it flexes more than their strictly all mountain designs. Mounted closer to center, smaller ski, with rocker = it will definitely flex when you charge. I'm larger than you and I notice the same thing (I have 186 Jeffreys for that reason), however my expectation seems much different than yours as I bought it as my go to ski since I rode 50/50 park and all mountain at the time. I absolutely love the J-mo, but if you don't favor a more park oriented design (like the J-mo and Jeffrey obviously do) then there are different options that likely are much better for your style.
As for how the skis measure, they do measure long relative to other brands because ON3P measures true length tip to tail while most brands measure the ski before pressing them. 5 long is a pretty aggressive recommendation however, as it may only be nearly accurate for the longest models.
I personally love Atomic and Armada as well, but I always seem to end up with ON3P filling out my quiver due to the success I've had in the past with them.
Slope Style Ski Shop, Main Street Breckenridge