So I ran the car in spec 17.5 this weekend(no boost) and it felt really good. It was a tenth or so faster than the X but like I thought you can run more consistant fast laps. The car feels the same lap after lap not changing much if at all during the run where as my X would take a few laps to come in every run and if I pushed too hard I would overheat the tires in the last 30 sec's or so of a run. Warmed tires, not warmed it didn't seem to make a difference other than the first 2-3 laps to gain heat in the tire without warmers.
I think it has to do with the motor mount more than anything else. The flex near the back of the chassis just feels more even and its a little stiffer.
All in all a great test day where I didn't have to change much from the stock set-up. The only real changes I made were to remove to shims from under the front blocks tighten the rear sway bar slightly and went down .5mm on the front inner link.