I am getting a 17.5 this week to start testing it. Our track has had some luck with it on the oval. The 13.5 is too fast for TC/12th stock. It's fun to drive, but realisticly far too much for a beginner stock motor. I hope we end up defaulting the 17.5 as "stock". The 13.5 can be "pro stock". We've found that on some of our layouts, the 13.5 is better than the 4300/10.5.
Anyone else doing any kind of testing? I worry about fitting the cars with enough gear to get the rollout. I've heard up to .500 more then the 13.5