Aaron, lots of good info above I'll try not to repeat but can't guarantee anything. I bought a 2015 390 in 2016 and sold it midway through this year and here's my experience. Prior to the 390 I was a big bike person with either liter bikes or big twins on track all the way back to 2011. End of 16 I got the 390 and in 2017 I spent a lot of time on it in the A group and it was fun, frustrating and a great learning experience. The pro's included
1 I was never worried about throwing the 390 down the road and it crashed well
2 Pushing to the limit of traction is not as frightening because speeds are down
3 Playing on track with other 300's in tight groups is a blast
4 Racing it was plain awesome both sprints and endurance
5 It made me a much better rider and helped me develop my cornering speed and technique much faster
6 It allowed me to feel the limit of tires and frames without getting in over my head
7 Super cheap compared to larger bike in fuel, tires, brakes and other wearable items
Lots of pro's and it truly was a fun bike some of the time, the Cons:
1 without other 300'srunning at my pace track days became somewhat boring and I would climb on my rc8 or r1
2 catching bikes parked in turns working to get past them only to have them motor past is frustrating to say the least
3 not passing people and being passed by lesser riders can be hard on your ego
4 Passing people is just fun and you won't pass nearly as many on a 300 especially early in your track day riding career
My recommendation would be this, if you're really excited to get out on a 300, do it. You'll learn a ton, and can get into the A group on a 300 but as you advance it will be come harder because your passing opportunities are more limited. There are many track prepped models that are selling for under 5K with tons of spares on the WERA/13x forum so you shouldn't have a problem finding one. I am 6' 205 pounds, these bikes are designed for 100 pound riders and the frames are made with spaghetti so if you're over 125 pounds know at a fast intermediate pace you'll start to get frame flex and start to reach the bikes limits in certain turns. If any of this is swaying you, find a used SV650 or older 600 track prepped on the wera board and roll out on one of them. Alternatively, get one of each and see what you like the most.
Tom