I had a 419x, and just switched to a TC7.1. While the x was a good car, I was immediately faster with the the 7.1. Can't explain why, because the x "felt" good on the track and I thought I had a good set-up. But when I put the 7.1 down, I was able to turn almost 1 second faster laps instantly.
I also thought the plastics on the x developed slop way too easily compared to other kits I've had. The 7.1, at least around here, has more support.