Depends really, I ended up putting an genuine 11.5 ltr tank on it as with the original 9ltr i was running out when my mates on 250's were just hitting reserve. I cane mine though

My local dealer said a lot of 300's are fitted with the 11ltr from new for trail riding purposes, the 9ltr's just for racing.
It's always run rich but I've just had the motor rebuilt and have spent a day playing with a box of jets. It's now a lot leaner so I should get more endurance, will find out tonight. Put it this way, with the 11ltr tank I have just a little bit more range than other guy's I ride with on their original tanks (250 2 strokes & a few big thumpers). You can get a genuine 13 ltr tank but I think you need a different seat to get it to fit.
Buzzing a long at 60-70 on the road really kill's the range, bimbling off-road it just sips it.
I did a hare and hounds recently and still had a couple of litres left after brimming it at the start and 3.5 hours riding
Whats the green spring then? You can adjust the exhaust valve via the screw on the R/H side of the engine casing. Wind it in to delay actuation and soften delivery.
I've tried to get as much poke as possible out of mine and it's still easier to ride than a peaky 200. You'll love it