Don't have any first hand experience from a stiffness point of view but I'd certainly be looking at a bolt-through for the additional safety over a standard 5mm QR.

Don't overlook the fact that the flex people talk about/notice is often in the wheel & not the fork. A good choice of rim & spoke well built into a QR wheel will be stronger/stiffer than a standard machine built bolt thru wheel.

the wheel point is true, but the stiffness i'm refering to is in the lowers, less leg movement relevant to each other means they bind less and the suspension works noticeably better over bumps in the corners etc. A large-flange 20mm front hub can also add wheel stiffness, all in all it tracks better than any 9mm set-up i've ever used – i'd agree it's not essential tho.

Could try a 9mm qr axle like the one from DT Swiss or Superstar Components.

I'd say there's a noticeable difference – I'm now running 140mm revs with 20mm maxle lite. I wouldn't go back to AR unless I had to (dropping to 100mm forks and no bolt through option?).

The other thing worth pointing out is that 15mm axle isn't officially a bolt through system as once the axle is screwed in, it doesn't get clamped into the fork legs on both sides the way a 20mm maxle or traditional bolt through does… which makes the whole axle, lowers, crown a complete stiff almost one piece unit. That's the point of the bolt through. The maxle system just made it quicker to get a wheel in and out, but it still clamps the axle in the fork lowers on both sides.

Hope that made sense.