A racquet with only a replacement grip (2,1 mm thickness) would feel the same size with a racket with a replacement grip (1,5 mm thickness) plus an overgrip (0,6 mm thickness)? Are the maths correct (1,5+0,6=2,1) or the overlaps add something more?
Keep thinking aloud because I've done a virtually identical experiment (via a grip buildup for 6'7" player), and the replacement grip (RG) combined with OG turned out to be half a grip size (1/16") larger in circumference than just RG alone of same thickness. I think the reasons are threefold:
1) There's space between the RG and OG, and that adds thickness.
2) Everyone but Andy Murray wraps their OG overlap-style, and that adds not just thickness but a thicker feel.
3) Grips compress with use, especially thicker (>1.8mm) RGs. If you're using a thicker RG alone you'll eventually feel that 'shrinkage' more than with an OG+RG combo because you'll end up replacing the OG (far) more often, which restores much of the original thickness.
Do leather replacement grips compress less than a synthetic ones? Weird that 1.75 replacement grips are considered to be thin, while leather grips go from 1.2 to 1.5 and are recommended to use to build up the grip size...
Yes, leather compresses far less than synthetics (especially the thickest ones), and it's far heavier too. So most players choose it to tail weight their racket as well as build up the grip. As to why 1.75mm thick synthetics are considered 'thin,' it's because the average replacement grip is 1.9mm. But I agree with you that 1.75mm is positively plush relative to most stock grips. For instance, the SkinFeel grip that comes on a Pure Strike is only 1.55mm. And the Syntec Team on a Pure Strike 100 G3 is even thinner at 1.5, Babolat even recommending it for those looking to shrink half a grip size. So replace the Team grip with a 1.75mm Syntec Pro and you effectively build up the handle a grip size (and lighten the balance a whole point too). And that's not conjecture. I did it for my niece's PS 100, and measured the differences. All to say, the key is to know what changes you're looking for in terms not just of thickness but weight, and then find the grip that fits. The good news is that there's a healthy variety out there, from thick & heavy to thin & light and every combo between. But don't take my word for it. Check out the blogs. There are a couple of great ones...