When I finally put both of them up on the screen at the same time (one above the other, in my main theater), I was able to make out the slight difference in blacks. I was mostly able to do it by comparing the two letterbox areas. The difference is not great, but is a visible difference. It's just that without a "side by side" type of setup, you would be hard pressed to convince someone. Take my word for it though, it's there, it's real, and it takes the best black levels and makes them better still.
In fairness, because the lamp on my JVC has about 600 hours on it, I had to stop down the manual iris several notches to balance brightness. Technically stopping down the manual iris should improve contrast slightly, but, the amount of drop in brightness I needed to apply to the RS25, shouldn't result in any visible improvement in contrast or blacks, to the best of my knowledge. (We're talking about only dropping brightness by 20-25%.)
Image time: First is a seriously overexposed shot of the starship in The Fifth Element. Note, that even with this ridiculous level of overexposure, the blacks in the image and the letterbox are still not dramatically brighter than black, if you compare it to the image right below. Immediately below it, is a less overexposed version, which is also better for comparing with the same image in older reviews. In this first image, I have left in part of the letterboxing, so you can see the basic black level more easily.