In stock form, mopar suspensions do create additional negative camber under compression and positive camber under extension, so he is right in that regard. Compared to some popular chevy and ford designs that create positive camber under compression and negative in extension, us mopar guys have had it pretty good from the start.

The whole point of negative camber gain is to keep the tread of the tire in contact with the ground as the car body rolls over. The more the body rolls, the more negative camber gain you want to have. So long as the tread face is in contact with the road, whether it is a 15" or 17" rim doesn't matter. The additional sidewall of a 15 may allow this transition to be smoother than a comparable 17 because the sidewall is a part of the suspension system and can absorb bumps better than a 17" sidewall, which can be an advantage. However, I don't think that I would say a mopar suspension will work better with a 15" than it would with a 17" simply based on rim diameter.

A 15" tire being faster than a 17" tire in his case is entirely possible because he is using competition tires with very rigid sidewalls. Stiffer than or equal to a 17", well, possibly, but I've never compared themside by side. I know 15" oval track racing slicks have a VERY rigid sidewall to them. Yes, a 15 slick is going to much faster than a 17" street tire. In competition tires, the selection of 15" tires is equal to and possibly better than in 17". Since there are very few companies that make both 15" and 17" tires in identical street treads, it would be tough to do a one for one comparison. However, if run side by side and optmized for each tire, a 17" tire may actually be able to use a softer wheel rate than a 15" tire because of the sidewall differences. Can't say I've ever tried it though.

Also consider that increasing the rim size adds to unsprung weight, so it is possible to have a 17" combo that weighs considerably more than 15" combo, but that all depends on rim and tire selection. I have seen some guys post weights of 17" wheels that were several pounds heavier than a factory rallye wheel, which is pretty heavy to begin with. With the Nascar spec, steel racing wheels he has on that Charger, he probably only has 18-20 pounds worth of wheel, which is incredibly light for a steel wheel.