I've been testing/running the around motor top deck for a few months. I ran it at snowbirds, as well as my local track and a few other places, that have more grip than birds. I've been running the 1.5 deck for a while, as I tested the 2mm at snowbirds and it was far too stiff(for there). I recently tried it again, and I found that it was a little better(more consistent and faster) than the 1.5 deck. The car felt like it was "springy" when it rotated in the corner. I raised the top deck .5mm and it seemed to really be a nice improvement over the 1.5 deck. This would be on a medium bite smaller track. I will be doing some more testing next weekend at Windy City.
Additionally I've been testing the rear anchor weight system. I really like this part on 99% of the tracks that I have raced on. It does a few things for the car.
I've gives it some more initial steering, due to the increased weight transfer.
It also gives the car more rotation, or stability (depending on how far out the weights are).
I did find on tracks that had high speed sections into 180's that you could potentially traction roll,or have the car become unpredictable in the corners.
The last thing that I've been messing with is the carbon fiber center spline. Since we have gone to horizontal top deck, we found that the regular brass spline can sometimes be too stiff. I found myself running just 1 screw in it, or taking it out. The carbon spline is a really nice tuning option.
It gives the car a nice consistency, as well as some additional stability in some of the high speed speed sections. I've been running it up 3mm, but recently I have been really liking a 1.5mm shim and a small shock o-ring under it. It adds some rotation, while not being "springy" (springy is what I've been describing as the car loads up going into a corner and then when it dissipates the energy it sometimes produces an inconsistent rotation for the car)