I find it funny that when you read threads about Dyno tuning on here, the peanut gallery jumps all over you because, "the Dyno doesn't tell you what will happen in the track" and "power on the Dyno doesn't mean fast on the track". So, now here is a guy that has successfully organized a regular event for 10 years for him and his friends, talking about pure laptimes and the peanut gallery is jumping all over him because they don't like stop watches and can't just agree to disagree with the posters battery preference? You really can't win on the internet. The stop watch times are fine when you are running a track with 30+ second laptimes. It sounds like a really large layout with a lot of scale appropriate flowing turns.
The poster has listed concerns about fire as his reason for not running lipo. How about someone addresses that?
In the early days of the lipo transition, many fires were caused by people trying to charge lipo batteries with non-lipo chargers and they were using ESC's that didn't have lipo cutoffs. I think the risk and occurrence of lipo fires got blown out of proportion due to that. The occuance of lipo fires at racing events these days is quite low. With a nice charger, a 2 cell lipo like you would run will charge very quickly, like under 30 minutes. Also, just to be safe, you charge the battery in a fire proof bag. This gives you a way to handle the burning pack if something goes wrong. You could charge your batteries outside on race morning and it wouldn't take that long. I believe after doing this a few times, you'd be much more comfortable with them. Buy name brand hard case batteries, not the direct from China stuff, a name brand charger, not a Chinese knock off, and a lipo sack, and you should have no troubles. I highly recommend protek batteries and chargers. I've been using them for years and have had great life and reliability and the prices are modest. I say all of this because some of the comments on here are true. The battery in your setup is the weak link, and variations you will see in motor performance will likely be due to their differences in performance in edge case regions that will make comparing your experience to other people's experience difficult.
Best if luck to you and keep at it.