I actually bought one instead of a 250cc just cause I knew I was going to upgrade in a year to a 650R. The bike is a little heavier than a 250R but not by much. I’m 5’10” weighing close to 190lbs., and I have no problems flat footing or holding up the weight.
The bike’s throttle is very good if you respect it. If you grab a fistful, even can happen on a 250R, it will take off. The bike just needs to be respected, but not feared. I feel just as comfortable riding the 650R as I did the 250 at the BRC. It’s all about how comfortable you feel. Sit on both bikes and see how you feel on them. If they let you test drive it, go for it, even if it’s just duck walking across the lot.
PS… I have no prior motorcycle experience before the BRC.