Welcome to the forums!
Sounds like you've got a good entry into the 300 with a track bike. Wonderful way to enjoy these bikes.
Indeed, the suspension is lacking on this bike, even if you're the weight they imagined when designing it. I am 150lbs and it is still soft. I did throw in thicker fork oil on the forks, and the stock springs are pretty close for my weight in terms of sag, so that actually made my front quite confidence inspiring. If you're looking to get er done for cheap, get some aftermarket springs in the appropriate rate and throw in some probably 15 or 20wt fork oil and the front will be happy. Especially since you already did the GSX-R shock mod. I think you should be able to get the preload and sag in the rear right with the stock GSX-R shock spring. Make sure the GSX-R shock has been recharged with nitrogen and I know you just put it in, but if it hasn't been serviced for oil and nitrogen lately, you should have it done, and you'll probably get away with it for years.
Got a pic?