My experience with prop overhang is with big stuff. Some of these things don't hit 200 RPM at a flank bell, they're big and very heavy, and introduce a significant bending stress in the shafting. Overhang on these things is critical as you don't want fatigue failure of the shaft inside the bearing (or anywhere).
Sailboat props turn much faster and are a lot lighter. The bending stress in the shafting is negligible even hanging out several inches. The concern becomes the whipping mode of the shaft line, which is tough to judge without a detail analysis. My 1" shaft is almost 6' long - the engine is up by the mast with the prop right behind the keel. I removed the intermediate bearing. The prop is aft enough to fit a full tapered zinc with no problems, but then it is a lighter weight plastic propeller from CDI.
As to your case, I'd be hesitant to let it hang out much further. You could try it, but be prepared for corrective action. Vibration due to the keel should be a minor issue since you have a 3 blade prop - only one blade is going through the keel wake at a time. I don't think another inch or two of shaft extension is going to do much for the wake deficit, and could certainly affect shaft whipping.