Yeah, the added thickness does not increase the inner diameter of the fittings and OD of the pipe.
Sched 80 is for real deal pressure applications. There is nothing on our tanks that would require sched 80. I use to lay sched 80 when I was building concrete plants. We were plumbing some serious stuff then (like 4 story tall chillers), so sched 80 was a must (county regulation).
Because sched 80 is stronger by build, with larger sizes you don't get as much splitting when you have to screw in threaded style fittings.
I would only use sched 80 fittings on the valves, unions and bulkheads. If I had the dough, I'd run all sched 80 just for the looks, but its not needed in this hobby.