I loaded for the 10mm for a long while but I got out of it. Brass cost and generally larger sized guns just didn't do it for me but the real kicker is how little difference there really is between it and the .40. Don't get me wrong, I'm not saying the 40 equals the 10mm in every way but when you really look, there's not a huge difference...although "huge" seems to differ between folks.
If you just look at much of the load data out there, the 10mm doesn't show to be much faster, and that's with it loaded to higher pressures and using a longer barrel 99% of the time compared to the .40.
Some won't agree with me here, but I think .40 brass is stronger than 10mm brass, the same as the 40K PSI rated 357 Sig. When the .40 was first introduced it was given the 35K PSI rating and early on there were a few issues with weak brass showing up, so basically the .40 was beefed up but the rating never changed despite this. Plus it's small primer pocket puts more material in the webbing of the case compared to the 10mm, which may not really be a significant boost, but more material means more support. To put it another way, .40 brass isn't any weaker than 10mm brass and is, in my eyes, a 40K cartridge like the 357 SIG. Others too have confirmed that the .40 can indeed withstand more pressure than the 10mm.
Whatever pressure the 10mm can handle, so can the .40 and then some. My reason with saying that is to make this point, if you used the same powder, the same bullet, loaded the 10mm and .40 to the same pressure and used the same length barrel, in most cases you would see less than 100 fps difference between the two. That's a fairly bold statement, but I feel it's absolutely accurate. (I'll post up a pic later to give an idea of what I'm saying).
I'm not convinced in the slightest that 100 fps (or lets say 75-125 fps to play it safe) makes any difference at all in performance. Pretty much with powders like Longshot and 800x and 3N38 for the .40 there's not much solid reason to buy a 10mm (well...that is unless you don't reload). My stock G35 will run a 180gr bullet to around 1,300 fps with the right powder, I don't need a G20 10mm to that.
Edit to add picture:
This makes a somewhat interesting point, at least I think it's interesting. This powder does well in both calibers as can be seen here, but as is with the 10mm loaded to higher pressure and using an inch longer barrel the difference is around 125-130 fps although the bullets are not the same design. I would say that if the pressures were made as close to the same as possible (assuming one had the equipment to test such things), in this case either dropping the 10mm 2300 PSI raising the .40 2300 PSI along with say chopping an inch off of the 10mm, the difference would be less than 100 fps, heck cutting off 1" of barrel alone would likely do that without messing with the pressure. Different powders vary but the differences seem fairly similar with other powders, and some like those from Accurate do seem to show more of a difference but I never could get claimed velocity with AA powders and the 10mm, nowhere near really. Just some food for thought.