This is a great tool for figuring out what gearing you want for a certain speed or rpm.
With as technical as grange is, I'd gear down so that you're in your power more thru the turns and coming out of them. A few years back when I went to grange like the very fastest bikes were barely touching 60 at the end of the straight. With that treat pipe you're probably revving out to around 10 or 11k depending on porti g and other variables. I'd set it up so you're topping out low mid 50s, with the chance to hit upper 50s if ya got a buncha room to stretch out. I'd go with 16*45 or 15*45. Just depends what you want, quicker pull thru the mids or higher top speed.
On the track I've been gearing my polini at 13 or 14*45, but that's with a simonini pipe. Needs that short gearing to pull thru the lows without overheating the clutch, but it revs high enough that it still tops out in a good spot for the track. On the same bike on the street I have it at 16*45 and it's a super fun street blaster.
I also bet lowering your seat would help you feel more comfortable on the track. I'm a shitty racer but posture and being comfortable out there makes a world of difference