I've been using a BA 'Sand Mountain' Insulated Air Core for a while now, while not a Q-core, its similar, so I might have some input that's relevant. Mine is only 2.5" thick, but I've never had any issues with my hips touching the ground (I'm a 180lb. side-only sleeper)I'm sure 3.5" thick is plenty thick for anyone. I leave mine about 60% inflated so I don't feel like I'm sleeping on top of a bundle of air tubes.
The Insulated Air Core and Q-core have the same 4.1 R-value: It's rated at 15*F, and I've slept fine into the 20's with a 30* bag and a down sweater. The pad was not the weak point.
All in all, It's pretty much as plush as they come.
It's also proven durable so far. It better be, it weighs about 50% more than a NeoAir or Exper UL 7.