I have both kinds, I'll see if I can free up some time to weigh them.
I don't know about 3 pounds, but I do recall the FMJ spindles being lighter to the point that it's noticeable if you pick one up.
It doesn't necessarily make them stronger though. The A-body spindles carry a thicker casting in areas that wouldn't improve the strength, just weight along for the ride.
First off- a camber, roll center, caster, and toe change chart from 20 years ago would be no different than one today if done at the same alignment and ride height. Assuming it was done properly. Nothing changed, the geometry of the spindles is same, the frame is the same, the suspension points are the same, etc. All of the parts in question have the same geometry as 20 years ago, so, all things being equal the chart would be too. If you have a suspension program you could make a fancier chart and plot, but beyond that they analysis was as extensive as it needs to be.
The camber curve was better on the FMJ spindles, by almost 1/2 a degree. The roll center was better with the A-body spindles, so was the toe change. But the SAI was better on the FMJ spindles. It's all in the Hot Rod article linked above, no need to guess.
The whole argument for the FMJ spindles was that if you're running wide, sticky tires the improvement in the camber gain would outweigh the increase in bump steer. Wide tires resist toe changes, and magnify the effect of camber gain. So on a handling car, the FMJ's advantages might outweigh the drawbacks. All the changes are pretty minor for a street car. On a track car they're not insignificant, but you'd have to be pretty dialed in to notice IMHO.
Again, the charts are published in the Hot Rod article linked above. So is the SAI and everything else. You don't have to guess, it's all there in black and white. The changes are relatively minor. It's not something you notice driving around town or by the "seat of your pants". It might show up on a stopwatch, but I would wager that there aren't very many drivers here consistent enough to prove it even against a stopwatch.
You seem familiar with the article, it lays it out pretty well. If you truly wanted to know how it would effect your car specifically you'd have to repeat that comparison yourself using your car with the ride height you intend to run and the alignment you want. All of those things would have an effect. The car in the article was lowered 1", so the numbers would change some if you ran lower or higher than that.
I've run both A and FMJ spindles on my Duster. It has FMJ spindles on it currently, I run 275/35/18's up front so if anyone would see a benefit from improved camber gain it would be me. I've only driven my car on the street, and in daily driving and even spirited mountain driving I can't tell the difference between the two spindles. Not saying that there isn't a difference, the numbers don't lie. But as I said it's not really a "seat of your pants" difference. I didn't notice any additional bump steer, so, that may be the most important distinction. I run my car lowered a good 2" from stock so my roll center is low to begin with. My car also sits slightly higher in the back too, so if anything my roll centers are probably closer front to back with the FMJ spindles. And that's an important distinction, you get a better balanced car if the front and rear roll centers are equal.
With the E-body, remember that the E-body spindles are in the same category as the B/R and FMJ spindles. They're already taller. Running FMJ spindles on the E doesn't change the spindle geometry any that I'm aware of if you're talking about 73+ E body spindles. The '70-72 spindles would be the same except for the smaller inner wheel bearings. The casting is different, so there is probably a weight difference between the FMJ and the E-body spindles. But as far as I know the geometry is the same. I ran FMJ's on my '72 Challenger as well, cheaper to get ahold of than anything else as they used to show up in the local wrecking yards.