I was running the lightest slotted CF paddles I could find. And the flybar really doesn't have "lag" so to speak. What it has is a lot of damping of the rate, which can make a flybar feel "laggy". But I can tune a flybar to be just as responsive as a FBL, and if you want to do extreme 3D flight with the flybar it is no problem at all with a "hot" setup.
No matter how light you run the flybar weight, it is still a rate controller. And you can't have two rate controllers working against each other. The answer to this in an autopilot is to be able to use feedforward and just bypass the rate loop. Without the feedforward you can't get enough servo travel if you turn the rate controller off in an electronic unit. And that is one thing that ArduPilot has that LibrePilot doesn't for helicopers.
The LibrePilot code is targeted to multi's, which cannot fly without a rate controller, and it just happens to work with FBL helicopters (with some other changes). I think it wouldn't be too hard to add this to LibrePilot. And I think it would be worthwhile to look into it and spend the time on it, being in commercial applications for successful UAV heli's they are all flybar, such as the Yahama Rmax and Fazer. The FBL rage is basically only for the 3D people, and has no real place in UAV heli's where utmost stability is required. Depspite all the FBL units, and even including LibrePilot with virtualbar, trying to mimic a flybar heli, none have succeeded in making one fly with the supreme stability of a flybar helicopter. Art Young's invention flew on every Bell helicopter from the 47 to the venerable UH-1 "Huey" for over 50 years. And despite modern electronics, is still used today in commercial helicopers where stability problems are not an option.
So as time permits, I intend to download and look over the code to see what can be done with it. All that is required is to have both the FC or pilot inputs to the attitude controller pass direct thru and let the flybar do its job so it can work in the autonomous flight mode. I did get the heli to fly but it was certainly not acceptable for high speed flight, nor for autonomous flight in any amount of wind after I flew and tested it a bit.