There aren't even any drop brackets in the SLR kit, they are offset bushings. The front diff is bolted to the chassis through 6 round bushings, about 2" in diameter. SLR simply offsets the hole in the bushings to drop the diff, an elegant solution.

Who makes the tougher a-arm ? Hard to tell, all the designs are burly.

The high angle balljoints are probably all sourced from the same company.

All the kits require preloading the torsion bars to increase ride, there is no other way to do it (without a full drop kit).
The new upper a-arm is designed with more room between it and the top bumpstop, so downward travel is not compromised.

By lowering the diff just a small amount (3/4"), SLR reduces the angle of the CV joint when it is at full droop, causing less wear and possible damage.

The SLR kit is more expensive, but you get a lot more for the money (Stage I, II or III).

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Regards,
Ian