I normally do both spares and baby back straight on the grates at 275 so 6 total hours of cooking time
Something is not right with those times at that temp. St louis spares should only take around 4 hours at 275, and loin backs around 3+ hours. Maybe you like rib jerky?
I'd verify that temps at cooking grate are what you think they are.
If you hang around here for a while, you will see some spectacular ribs that were done in less than 4 hours cooked naked. No spritz, mop, foil, or peeking. Just let 'em rip till the sizzling sound starts to quiet down.