I tested my D815 by starting with my D812 base setup exactly except for the new gearing. Tested to see how the new gearing feels. Then I made one change at a time to all the new D815 parts including front hubs, rear hubs, etc.
I think there is a big difference in a great way between the D812 and D815
Much smoother throttle feel, better brakes, more consistent shocks, and I really like the 17.5 front hubs. Steering was fast, smooth, and stable. On a really loose surface, I like the outer lower hole on the D812 rear hubs, so I keep a set of D812 rear hubs since I can't replicate this setting with the D815 rear hubs.
And on top of the better handling, the better servo saver (beefed up, no longer loosens over time, and no longer rubs the front center cvd), split center diff, and no more failing cvd pins in the center driveshafts makes the D815 that much better.
So far, the car has been great.