I've got a 40D APS-C with a 1.6x crop factor. That means my 70-200 is actually x1.6 or 112-320mm.
No, the 70-200 is still actually
a 70-200. Mounting it on an APS-C sensored body gives it an effective
focal length of 112-320mm.
Given that i'm shooting at 10Mp, if i upgrade to FF a 5Dmk3 given the grater number of pixels how would I calculate the zoom of a 10Mp image cropped out of the center? I'm thinking it's greater than 1.6 but not sure how the math works out.
You need to know, or figure out, the size of the pixels. The 40D has a sensor of 10MP, measuring 22.2x14.8mm, or in pixels 3888x2592. So, to get the size of the pixels, you divide 22.2 by 3888, giving you 0.00571 mm, or 5.71 microns (or micrometers). You can do the same for the vertical measurement, but since Canon uses square pixels, it's really not necessary.
The 5DIII has a sensor of 22.3 "effective" Megapixels, on a sensor 36mm x 24mm. Largest image size is 5760x3840. We can do the math, but Canon very nicely publishes the pixel dimensions on the 5DIII spec page
as 6.25 microns. So, the 5DIII pixels are larger.
So you want a 10MP, 3888x2592 image out of that? 3888x6.25 microns is 24.3mm, and 2592x6.25 is 16.2mm, about a 10% larger dimension for the 10MP "crop" out of a 5DIII. On the other hand, a 22.2x14.8mm crop gives you 3552x2368 pixels, or about 8 MP total.
If you are looking to upgrade, what is it that you shoot? The 5DIII is a great camera, well suited to many things, but if the effective focal length matters that much to you, maybe you would be better off upgrading to a 7D, which would give you nearly double the number of pixels, and keep the crop multiplication factor.