I shoot what I'll refer to as "casual" F-class. That's my setup though I'm not a hard- core competitor. I recently switched from .243 to 6.5 Creedmoor.
I don't clean the inside of the brass, other than inside neck brushing. The outside gets a light scrub with 0000 steel wool, and the primer pocket gets brushed. Then the brass gets resized and trimmed if necessary.
A few months ago I loaded several .243 test rounds with 8x fired brass and compared velocity with 1x fired brass with the same bullet and powder charge.
There wasn't enough change in speed over a chronograph to note any difference at all. I haven't tested with the 6.5 yet.
I will say that you should be careful if humidity is high, be sure your brass is dry before you load. I've never had problems with dampness but I could see it being a potential problem under very damp conditions.