Supermoto on a road course correct?
You'll have a real hard time on the straights at 15/45. That's more suited to kart tracks with short straights or off road on 17's.
My bike has the stock exhaust, it would do 75 mph easily with 15/45 and it touched 85 mph once, but it was not liking it at all. It was fun, but way too short gearing for even public road use.
16/45 will cruise nicely at 75-80 and do 90 without wringing it out too much, and will just make 100 I think. It is a good compromise for road and off road use, the gear spacing is ideal with no awkward gaps. I've ran 16/45 for a long time now.
I'd still like to try 16/43, I think it would be good for road use but could result in speeding tickets.
It had the factory 16/40 when I bought it. Made for awkward gaps in the tight twisties. Either constantly shifting 3-4-3-4 between the many turns or just keeping it in 3rd and letting it rev. It was ridden as a 5 speed in the twisties, and in town as 6th was unusable until ~70 mph. Big gap between 5-6 with 16/40.