I just picked up a box of this stuff after seeing a few sample prints. I'd been looking for a nice matte paper for my black and white prints, and it was hard because I have generally disliked every matte paper I've ever seen or used. Most of the cheaper matte papers feel like I'm printing on cheap cardstock. Some of the better matte papers are 'close', but not exceptional. A bit too washed out or lacking that exceptional fine detail I get with my two preferred semi-gloss papers (Hahnemuhle FineArt Pearl and Red River San Gabriel Fiber). I liked the Hahnemuhle Photo Rag papers, but the plain photo rag has a somewhat sterile rendition that is good but not great, and the photo rag bright white looks really nice but uses a ton of OBAs, which I reticent to use.
The Canson Rag Photographique, though: wow. Brilliant black and white prints, wonderful texture and feel, outstanding contrast, very deep blacks for a matte paper, and the color prints are equally stunning: such rich color and simply incredible detail in the final prints. Absolutely wonderful matte paper...making me a convert for using matte. Definitely will be using it for a large portion of my B&W work, and I really like it for color work for certain images, though not all. Just really impressed.
Canson Rag Photographique; 310 or 210 gsm? The 210-gsm shows more reflectance in the blue wavelengths than the 310, which should make it slightly cooler - perfect for my mountain color landscapes and B&W snow-on-trees images.
I haven't seen this while profiling both papers. The 210 is more prone to influence of any backing material, being less opaque than the 310. If you are using the Canson provided profiles, the 210 profile definitely is skewed noticeably cooler, but that isn’t due to the paper, it is the profile.
Not using either one yet, just researching which to get.

Red=Canson Rag Photographique 210-gsm, Green=310-gsm (SpectrumViz.jar)
Note the slightly blue-shifted 210 compared to the 310.
I just installed the Canson RP 210 and 310 profiles and see a slight difference in the LR histogram between the two - with the 210 blue regions shifted to the left (bluer) compared to the 310. Visually I cannot discern a difference in the soft proof screen appearance of the two, but obviously the histograms say it's there.
You said you don't see this in your own generated profiles and that the way Canson performed the profile creations is what is causing this blue shift in the 210. I assume you attribute this to the 210 being thinner paper and therefore more prone to background bleed through during the actual scanning process - where it wasn't done quite right?
Why couldn't the blue-shift be caused by both the bleed-through (Canson incorrect profile creation) as well as the natural bluer reflectance of the 210 paper itself? Both factors would end up showing a blue-shifted histogram and slightly bluer paper.
I guess I'll just have to wait until I print both the 210 and the 310 to see if there's a significant difference and whether or not I prefer one over the other. BTW, that 310 is a lot more expensive than the 210, so it would have to be a big difference for me to spend the extra $$$.
The Rag Photo 210 is my next Canson paper to get. I already love their baryta, Platine and especially the Aquerelle Rag 240-gsm.
There are really no not-good Canson papers. I just wish someone would come up with a cotton paper with the texture and resolution holding properties of Hahnemühle German Etching, without the OBA content, small as it is, I’d prefer a more natural colored paper.
I got some Canson PhotoArt HD canvas to play with and was not impressed with it compared to BC Crystalline White. Otherwise, all the other Canson papers I've tried were either fantastic or better.
The Montval and Etching papers are also excellent. The Rives BFK has been around for at least a couple of hundred years, in a non-inkjet coated form, and is one of the best velvet textured papers around.
I'll try that Rives BFK after the RP 310 and 210.
If you like lighter-weight papers, then Epson’s Velvet Fine Art paper is very, very good. A fine paper in the Japanese tradition of quality over weight.
I actually prefer heavier weight and/or thicker papers and was only looking at the lighter weight papers so that they would feed through printer rollers (ipf6400) better and be easier to control the curl. I'm not too experienced with this WF roll printer yet.
This passion for fine art papers is becoming expensive, but I think I'm becoming addicted.
soloryb