Typical full fuel payloads on SR20s coming out of the factory are 520-540 lb I believe. That’s with full fuel: 56 gallons usable. So to carry 600 lb in the cabin you’d need to have a maximum of about 46 gallons on board, for 700 lb in the cabin, only 29 or 30 gallons. 800 lb is a non-starter (all weight above 2900 lb has to be fuel) unless you’re just making 1 trip around the pattern, and even then you would be above maximum landing weight!
I think (correct me if I’m wrong, guys) that a typical LOP fuel burn is 8.5-9 gph, in exchange for which you’ll get 140-145 KTAS. Pretty efficient! If you depart at MGTOW (3000 lb) with 30 gallons and want to arrive with a ~1 hr reserve, that’s ~2 hr cruise endurance, and you’ll be right about maximum landing weight (2900 lb) for arrival.