For printing on canvas I doubt if you would see any difference at all. Canvas is inherently a low gamut media that would not much strain the abilities of either printer.
That's assuming you managed to pick up a cherry 8300 where both heads were young and perky, the carts averaged more than 1/2 full, the maintenance tank was low mileage, there was no visible duct tape, the 16 monitored sub-systems will still all "OK", blah blah blah. At this point in time, good luck with that. Printers get replaced for reasons, some of which are not pretty.
I just this week swapped both heads on a high mileage 8300, and wow! It's working like a champ! Mainly because I have always treated Jolene like the lady she is and never ignored her. But the only reason I didn't pick up an 8400 was logistics made doubly daunting by having to both remove and replace a giant printer with outstanding jobs in the wings. I'm sure you have heard that a new printer includes a $900 pair of mint print heads, and $2,000+ worth of carts, most of which will still be available for printing after priming, and of course that irresistible new printer smell. For around $3300. OTOH, if you have some time available, you might get lucky with an 8100. But to be strictly price competitive they would have pay you to haul it away, and you should be ready to dump it at the first sign of trouble. And I'm sure you know moving an ink-primed printer is no picnic.
Here's a thought...my 8300 can download a file containing its current status, including the age and status of the heads, etc, plus obscure error codes describing what kind of issues it has been having recently. Check those codes against the service manual, see what's happening. Run the "ImageProGraf Status Monitor" located on the host computer, go to "Information", and select "Status Display." Copy the text to Nopepad if you're in Windows, or whatever works on Macs.