Re: why not a 4.5 inch lift?
I'm sure people have plenty of fun wit 6" lifts, but In my estimation, they're primarily for looks and only secondarily for wheeling. Even then, it's because some customers would demand that they not hobble suspension performance, while many others would buy them for looks and never notice.
The point is, nobody's ever posted up any facts beyond customer satisfaction testimonials, and since there are obvious system concerns, it's up to the people professing their effectiveness to put up the data. Maybe it's out there, but I haven't seen it.
The height of the center of gravity is one of the most important determinants of linked suspension behavior, regardless of whether or not increased height is an increased risk for flops. If you raise it up 3", the rear lower links are now at a steeper angle, then the antisquat and the roll axis have both been significantly altered. If the roll axis now has a positive inclination (I don't know that it has, but I suspect this), then you'll have gone from a balance seeking to a balance shunning link behavior.
From what I understand, if the roll axis runs "downhill", then the chassis will tend to self-right in the same way that a refrigerator door closes. On a fridge, the hinge mechanism actually lifts the door as it swings out so that gravity bringing it back down tends to swing it back closed. The same is true of a well designed link suspension. Gravity tends to center it.
If the roll axis goes uphill, then the opposite is true. The low points are at the ends of the swing and the high point is in the middle. It tends to seek a leaning state and requires mechanical opposition.
Also, the steeper the rear links at ride height, the more there's an effect of rear steer. When the axle goes into twist, the wheel that goes low also hinges forward while the wheel that goes high, swings back. This changes the direction the tires are pointing, relative to the long axis of the vehicle. They start to steer.
The FJ Cruiser doesn't have a lot of suspension movement, and most of it's twist capability happens with rear suspension articulation while the body tends to parallel the angles of the driving surface that the front suspension is on. Because of this, the rear suspension geometry is even more important... and almost completely overlooked when people start talking about suspension kits.
I've never once seen anyone use link calcs to evaluate the kits that they think are so great. That would probably put a lot of this ongoing blah blah blah to rest.