This is a project I’ve been working on for a long time. But I had to find a densitometer first, and that took about a year! But now I have a great little thing (review coming soon).
There is a lot of speculation about these three films, Kentmere 400, Rollei RPX400 and AgfaPhoto APX400. The consensus seems to be that they are in fact the exact same Kentmere emulsion just repackaged. So I wanted to look into it a little, and did tests on all three at the same time and developed them all together. My XTOL is over a year old and it seems it has started to lose some potency, so the negatives are slightly underdeveloped. That would explain that I am just getting ISO350 instead of the rated 400.
The fact that I am getting the same contrast and speed on all three films is a giveaway that they are in fact the same emulsion. The shape of the curve is also more or less identical. I am using my enlarger to expose the film, and the bulb has a slight warmup/cooldown time, and my darkroom is not 100% perfectly dark, so the very small variations that are there - I expect to be due to this.
So to summarize, my conclusion is that they are the same emulsion.