So far, I haven't seen much difference in gloss, but that probably has to do with the fact that I virtually exclusively use etching papers as a final support. I generally use Schut Laurier, which is a pure-white, 300gsm paper with a surface that's somewhat coarse/fibrous but not with the kind of texture you often find on watercolor papers. I size this with gelatin, fairly heavily as well, at around 25g dry gelatin per square meter (this translates to ca. 5% gelatin solution poured to a height of around 1mm). Hardening I generally do with chrome alum added directly to the sizing solution because the paper is ready to use as soon as it's dry.
I've transferred to other kinds of papers as well; lately for testing purposes sometimes to Yupo and in the past occasionally to RC paper. I never use fiber paper because I don't have a reliable supply of expired FB of the same type and size, and fresh paper I prefer to use for its intended purpose
I don't care for carbon transfers to 'high-tech' surfaces like Yupo and RC paper one bit. For me, it takes away pretty much the entire reason for alt. process printing, which is the possibility to use papers (and other surfaces) normally not accessible for (analog) photography.
In any case, where this is going is the observation that the more heavily sized the surface, the more critical the surface quality of the tissue becomes, I think. On the papers I size myself, there just isn't much difference between the paints or pigments I use in terms of surface sheen, because the influence of the paper texture and sizing are dominant. With FB papers, this would already be a different story and with stuff like Yupo it's a totally different universe altogether. But to be frank, I really like the semi-matte/eggshell sheen that the Laurier paper gives me. It's really a very nice optimum between a 'natural' paper texture with its inherent fibrousness and the extremely high resolution and detail that a smoother (hot pressed, heavily calendered or even polymer) surface gives.
I really do understand the attention you spend on this. For me, surface finish is one of the most critical aspects of print quality. It's also one of those things that people accustomed to inkjet prints for the most part simply aren't aware of to begin with (the same goes BTW to people who only print on RC papers). But it's such an exciting aspect! It really makes the difference between a technically good print and a beautiful one.
PS: concerning inks, and black in particular - black is just such a challenging thing to get right. I think I've said before that I'm not too fond of being stuck with warm blacks such as lamp black, ivory black etc. But fact of the matter is that
only these blacks tend to disperse well in an aqueous solution. The most accessible alternative of a more neutral hue is iron oxide black (PBk11), which is a popular pigment in all kinds of paint, for instance Winsor & Newton's 'Mars black' watercolor and gouache paints. But the problem is that iron oxide tends to drop out of solution, which is a major concern if you have a jar of glop outgassing for a couple of hours (or overnight, as you've been doing). Also, it's magnetic, which I became painfully aware of when I tried using a magnetic stirrer once to effectively disperse an iron oxide pigment in a watery solution; all the pigment ended up stuck magnetically as two soft balls to both ends of the stirrer bar!
Just now, I was doing a little tinting strength test with Winsor & Newton Mars black (iron oxide) watercolor vs. Talens India ink (lampblack, most likely, in any case a fine carbon black):
Winsor & Newton iron oxide black to the left, Talens India ink carbon black to the right.
Note the difference in hue; the iron oxide is near-neutral, the carbon is very warm.
Also note tinting strength; the jar to the left had 20 drops of a 1% solution added to 20ml of water, the jar to the right only 8 drops! I think it's mostly due to the much finer particle size that the tinting strength of the carbon black is dramatically higher.
The matter of black concerns me at the moment, because I'm kind of stuck with carbon black for the key layer in my color assemblies for now, but I really think it's far too warm toned for this. Effectively it creates crossover this way, which will be noticeable (warm shadows). I might just have to deal with the iron oxide's propensity towards settling at the bottom of the jar and stir frequently as the glop is outgassing. Such are the woes, eh?