Are you saying that the 8000's will work in a 5si (and vice versa)?
The 8000 series uses the same Canon WX engine as the 5Si series. Thus,
a lot of hardware parts are interchangeable. A duplexer, for example,
can be swapped back and forth. (When the duplexer in my 8000 started
giving me trouble, I used the duplexer from my 5Si in it for a couple
months until I could score a replacement at a good price on eBay.)
Most hardware items are interchangeable. The 5Si and 8000 use the same
toner, same maintenance kits, etc. But the 8000 has newer electronics,
so things like the control panel are not interchangeable. They also
use different JetDirect cards, and the slot they fit in is smaller on
the 8000 than the slot on the 5Si.
And what I was saying about the trays is that, even though they are
hardware and you'd think you could swap them, you cannot. I was having
trouble with tray 3 in my 8000, so to find out if it was the tray or
something else, I decided to put tray 3 from my 5Si into the 8000. It
wouldn't go in. The two trays look identical, but when you look
closely there is a little thingamabob on the back of the right edge of
tray 3 which tray 3 from the other printer is missing.