If you look at BR's and FA's PPs for the Derby, they have fairly similar running lines. They were both relatively close to the blazing pace and didn't finish that far apart at the end. The major difference in their trips was the gound lost.

BR was being used harder to stay with that pace while losing ground on the outside.

FA did a lot of his racing on what appeared to be a dead rail on KD day.

IMHO, formulizing the pace (that they both contended with) and the ground loss vs the bias into one comprehensive assessment of performance is no easy task.

Even if the rail was dead, how dead? How many lengths did it cost?

Was the 3 path better than the 2 path or were they equal. How about the 4 path?

Even if BR was on the better part of the track, does that offset the fact that he was being used much harder chasing that fast pace around turns on the outside?

I don't think there are clear cut answers to these kinds of things that can be measured precisely.

I think you can know that both horses ran WAAAAAY better than it looks on paper.

I think you make attempts to estimate impacts by looking at how other horses in the race (and other races for insights into the bias) with similar trips and ability performed. The answer will remain very fuzzy, but like I always say, I'd rather be approximately right than precisely wrong.

I think FA's performace in the Derby was a clear cut peak effort up until that time.

I think BR's performance in the Derby wasn't quite as good as his Wood, but it was darn good. I think this is a classic illustration of how much of an impact perfect conditions vs. horrible condition can have on speed figures.