Two things that make the question moot:
- Your EOS Rebel XTi/400D is only rated to autofocus with lenses f/5.6 or wider (lower f-number). There are third party lenses with f/6.3 maximum apertures at the longest focal lengths that still autofocus on Canon f/5.6 limited cameras. Many have theorized that either the third party lens "tricks" the camera into thinking it as f/5.6 lens currently set to f/6.3 or that in this context f/5.6 really means "anything wider than f/8." (f/8 is the next standard "full stop" past f/5.6)
- Every 500-1000mm f/8 lens I've seen is really a 500mm f/8 lens with a converter attachment that makes it a 1000mm f/16 lens. They're also all manual focus. And not very good optically...
For more on the whole "aperture limited" thing with regard to autofocus, you can read this answer to Comparing Canon EF 100-400mm f/4.5-5.6L IS II USM and Canon 70-300mm 1:4-5.6 IS II USM with Kenko 1.4x MC4 DGX
There is a difference between being rated to focus at f/5.6 as most of the older Canon cameras are and being firmware limited to f/5.6 like some of the newer Canon cameras are. Older Canon cameras rated to focus with f/5.6 or wider lenses will sometimes focus at f/8 depending on conditions. But that ability will be fairly limited to very bright light and will be excruciatingly slow, especially with cropped APS-C cameras. I once tried it with a Canon EOS 7D + Kenko C-AF 2X Teleplus Pro 300 DGX + EF 24-105mm f/4 L IS. The only thing it would AF on, and then only some of the time, was a bare light bulb using the center AF point.