Did you check the front knuckles of your old zip for debris?
What wheels do you have on the new zips?
I found that if there is crap in the knuckles it slows down the zip alot. My first zip with the nos would do a doughnut (kinda) and was fast but I didn't get flipping speeds until I did a pcb swap.
There will be a couple of stock zips tonight that are never touched and I'm picking up another new zip tonight (hopefully) and there will be some serious testing. The way these cars are manufactured there really is no way that 1 zip would be faster than another stock so there must be others. Here's hoping that Rs finally wised up.