Wishbones vs Struts
 

How much (if at all) better would double wishbones be for the 86 assuming you can get about -3° negative camber and limit body roll with a stiffer FSB on the struts?

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Ignoring the big hunk of an engine in the way of the upper wishbone? It would probably allow for better handling but I doubt it would be something mind blowing, particularly if you're being realistic in your thinking (such as using a Toyota I4 instead of a Subaru H4 from the start) as this would still be a budget car.

Your -3 degrees of camber is unrealistic to expect stock. In all the front DWB cars I'm familiar with, stock camber adjustment is far from enough for serious track use and all of them have aftermarket adjustable wishbones to cater for that use. Sure, DWB offer a much more favorable camber curve over MacPherson struts, but street cars typically lean so much into corners (relative to proper race cars) that the dynamic camber gain of DWB over struts isn't enough for track driving.

I've actually pondered a similar question to yours, and my conclusion is that any component comparison on a car is not black and white (in terms of "with DBW it would be better") but rather need to be treated more as a scale, where perhaps DWB can be potentially better than strut, but well sorted strut suspension can be just as good as well sorted DWB, and better than poorly setup DWB. One of the best handling sports cars (the Porsche Cayman) has struts on all four corners. Is a Lotus Evora so much better for having DWB all around? Or is it a matter of tuning and decision making when designing the car? Would the Evora really suffer that much if it had to use struts?

You'd need camber plates to get that much camber of course. Like you said there are lots of cars which are considered to be some of the best handling cars in the world (M3, Camaro, Cayman, 911) and they use struts. Surely they're not so much of a disadvantage assuming you can limit body roll to as many degrees of static negative camber you can get on the car, if they are at all.

The DC5 Type R is unequivocally faster than the DC2 despite struts vs wishbones.

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One thing to note with the list of best handling cars in the world there - you can get a look where they are going. Double wishbone up front for the M5/M6 and 911 GT3, likely to trickle down. Also note that all are road car based.

No doubt wishbones are superior to a 0 camber rolls-like-a-boat OEM strut setup, however I see no reason why you can't equalize the strut setups with lots of static camber and limiting body roll. The GT3 is sold as a track ready car, I doubt anyone buying these or M5s want to screw around with geometry. Struts are a way to bring costs down

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It's good enough. With wishbones it would be easier on the tires and you may get a bit more braking stability from being able to run less static camber. I bitch about it but most of my issues are my own doing and I doubt I would gain much if any benefit.

Packaging is also a reason for macstrut. That boxxer causes a wide engine bay.

Yea tires will suffer with -3° unless you heavily track the car. But braking shouldnt matter as much because when you brake the weight of the car gets flung forwards loading the front suspension and increasing negative camber which will hurt braking in a straight line (on a wishbone car)

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Keep in mind that until the 992 generation the GT3's shared the same strut front as regular 911s. The Cayman GT4s also didn't change the base strut. Both models are lauded as excellent track cars stock. Also, from the reviews of the 992 GT3 it is apparent that this was a change for the better, but not a huge difference.