I have been in the VW hobby on and off since 1974 and always been of the understanding that with it's torsion bars adjusted properly, a VW (especially a street car) doesn't need any more spring, front or back. The odd time I've seen coilovers they've been cheap blister packed products on a baja bug or offroad buggy, most rode like crap and there was usually the question of whether simply adjusting the torsion bars for the ride height would do the trick and provide a better ride. The exception would be a heavier modified baja bug with bigger back tires where instead of the cheap springs on the cheap shocks that are usually available (which would provide the lift that was looked for but rode like sh*t), slightly thicker Sway A Way torsion bars and decent shocks would be much more appropriate.
If one was to eliminate the torsion bars completely and go to properly tuned (note those last 2 words) coilover/shock assemblies I see it, but don't know if there would be any measurable advantage on the street. Kevin at Coolryde Customs has done this with his Mendeola front and rear suspension systems and claims far superior ride and handling (which in part is probably from the wider wheels/tires used at both ends) and much easier adjustability, but at $6,000 ($3400 front, 25-2700 rear) plus installation and more for wheels and tires to get the most out of it, not many replica Speedster owners have made the jump. I could see going with the front A-arm assembly, but I'm not convinced that his rear suspension would be that much better than a narrowed irs/wider tire/properly shocked set up.
While I've never heard of anybody breaking the rear shock towers (not to say it hasn't been done, I've just never seen it), for any performance driving a kafer bar tying the tops of the towers together and triangulating down to the frame horns and maybe even the torsion bar tube is a good idea. And if you do a kafer bar and are worried about the added weight you could do this
it doesn't totally negate the weight gain, but I did remove just over 1½ pounds per side and (I think) it looks really cool!