There is really no perfect geometry for all cars since each level of performance and combination can vary so widely. What works for one car doesn't always work for another, even if the combo is almost a perfect match.
I go by a couple rules that seem to make life easy & get results with the least negative side effects:
Keep it simple! You can think yourself to death & try all kinds of things, but usually the simplist of ways turns out to be the best.
Try to make what you have work before trying something new. I know people that have gone mid 1.4's with stock un-boxed control arms with stock bushings! Can we all do it, certainly not. Could the combo have gone quicker with better suspension, by all means. It's all relative. A 9 sec combo can come out fine (for most of us) with stock setups, but won't be in line with other 9 sec cars or be as fast as they should or could be. Don't give up on what you have too soon.
Matched setups work best. Many people use brand X lowers with stock uppers or brand Y uppers & bushings in the top ears from brand Z, and wonder why it doesn't work right every time. We have been getting even better results from those who get our complete setup. Our stuff is designed to compliment & work great with any other manufacturers parts, but other manufacturers parts are not always the best thing to use. There are a lot of well known brands out there, and many of them have short comings that reduce or inhibit performance to some degree. A simple thing like installing new control arms with grease fittings all the way around, and either leaving the stock bushing in the upper ears of the rearend housing, or installing the competitors poly bushing (with NO grease fitting) that has twice the poly of the matching components. Now that one position out of the 4 will restrict movement & take away from weight transfer. Bunch of money spent, slight increase in performance, not what it should or could be performance-wise.
Bottom line, lowering the mounting point on the rearend for the lower ca's makes the instant center more appealing (by the book) for racing, but makes the car handle worse at high speeds when it is unloaded or under braking. Some may find it just works better for them doing that, and they don't want to change anything else or take time to tune it in. As said above, we can get a simplier setup working better, without the side-effects. The IC bracket idea just works a little better on some cars because it is a band-aid fix for something else not working right. They seem "at-home" on 12 sec cars that don't have enough power or converter to get a binding suspension moving quick enough. So instead of fixing the binding issue, the IC brackets provide relief. On higher HP cars, we are seeing a trend going the other way. Lowering the FOREWARD mounting point of the lower ca's, which puts the intersect point way our BEHIND the car instead of inside or in front of the car. This is because they have to tame down the geometry so they can apply full power on launch. With a suspension setup that is fine for a 11 sec car and an 9 sec motor, the car would do a wheel stand & flip over.
