The backfire is kind of normal for straight pipes on our rich running vulcan engines. Marbling should fix that cheap and easy...
As far as the rest of the questions:
How much fuel do you have when the dummy light comes on?
From my experience, right around a gallon.
About how many miles do you get per tank and/or whats your mpg?
I haven't run the full tank yet... I usually just fill up somewhere between 160 and 200 miles on the trip meter. My mileage averages anywhere between 46 and 51 mpg, depending on how hard I roll the throttle. Normally 48 on most fill-ups. For a 6' 4" 250lb man, I'm happy with that. I've also taken several day-trips riding 2-up with my wife, and I've gotten 47-48mpg on those trips as well.
Does the owners manual call for 87 octane or higher?
The OM calls for high octane, yes. There are folks who run regular unleaded in theirs with no reported side effects, but I'm a firm believer, if MOM says it, there has to be a reason. I *highly* recommend finding a station that sells their fuel Ethanol Free as well!
How does the monoshock compare to traditional shocks on ride?
I've put in 4300 miles since 4/24, and I have no complaints about the ride at all. The stock seat could use some tweaking, but the ride is fine!
Hope that info helps! Let us know when ya get it, and share some pics!