I fly a mini-Softie backpack in my 6, packed diaper down to give some lumbar support (rear seat cushion removed, chute against standard seatback.)
Without the diaper-down rigging, my lower back was killing me after ten minutes.
I also had a sheepskin back pad added for additional comfort.
I have the aerobatic style harness, but that was on it when I got it, so I have no experience with the standard harness. I also had the straps shortened when the pad was added as I had a ton of excess strap to deal with.
I like it. I fly it most of the time I'm up. I think the longest leg I've flown with it is about two hours. Not as comfy as the AeroClassics seatback, but not horrible. It does push you forward an inch or so(?), but I got used to that quickly. Not really an issue.
Mini-Softie, diaper down, sheepskin pad, aero harness.
RV-6A. Mattituck TMX O-360, FP, GRT Sport EFIS, L3 Lynx NGT-9000
N184WM reserved (RV-8)....finishing kit in progress. Titan IOX-370