I don't think tuning is going to throw you off 9 freakin inches. Even with a bow severely out of tune if you have good form you should be able to group arrows fairly well at 20 yards.
Inconsistent shots are usually something being loose or poor form. If your other bow shoots well for you I am at a loss without seeing the bow and you shoot it. The quick tune 800 is a pretty simple rest. The only thing I could think of off hand that might be a problem would be rest tension, and that would effect up and down misses, but would still be fairly consistent unless you were getting fletching contact.
Unless maybe your spring tension is too weak and the arrow never comes all the way up to the same spot at full draw. If this were the case though when you nocked your arrow the rest would drop from the weight of it. If it doesn't do that I don't think that would be your problem.Possibly if you don't use a string loop there might be enough tension when you draw the bow to force the rest down some?
Check and make sure your rest is solid and not moving around from shot to shot, or your sights. These would be the two biggest culprits of misses of that size. I would also double and triple check that you don't have fletching contact with your rest, shelf, riser or harness and that your arrow are all indexed the same.
I would seriously find a good shop and tell them what is happening and see if they can figure it out. That big of disparity in groups should be something pretty obvious. I have shot vanes off from flipper rests and got more consistent groups than that
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I really don't think it's something as simple as something being out of spec or slightly out of tune. You should still be able to shoot pretty well with an out of tune or out of spec bow. I have done it on purpose with little effect when using target arrows. Now with fixed blades it makes a big difference though.
I'm really curious to know what is wrong now. Where do you live?
Paul