An '87 Zilla with the slower external gearing was verified to go over '79 mph years ago in 3&4 wheel action.
About 10 -15 years ago, My bone stock '88 zilla out ran a Honda XR 350 enduro bike on the street, even at top speed. We were trying to use the enduro to check the top speed of the zilla. (We didn't have GPS back then.) The first time we ran, I took off and he never caught up so he couldn't tell me how fast I was going. So the next time, He started out about 1/8th mile behind me, and I waited for him while he got up to speed and when he got close, I took off. He still never quite caught me. At one point, he was within a couple bike lengths but I was slowly pulling away. His speedo was topped out just a tick over 80. I was going roughly 81 or 82 according to that if the speedo was correct.
And an FMF pipe on a zilla is junk IMO. The FMF's for 250's are ok, but I'd rather have a stock pipe on a zilla compared to an FMF.
I also new a guy years ago that had a zilla and two YZ250's. He said the zilla would spank them both. I couldn't believe until we rode them in a grass field. Holy crap, the zilla was fast. He was right about it beating the YZ's. It was a time bomb, but it was fast as hell. (Idling, it would overheat and leak antifreeze.) I think it had an LRD or trinity pipe. Nothing special by todays standards.
If a 250 four stoke is even close to you, you have serious issues with your zilla and your loosing the traction war. Race on pavement or hard packed dirt to make sure your bikes at least outrunning his. Then you can try different tires / gearing to figure out how to win in the loose stuff.