Something no one has pointed out: Buff-hunting would be more fruitful if there were no 2x fuel burn multiplier. As things are, climbing up to 20k+ to search for bombers leaves you little fuel in a lot of the good bomber killing aircraft, when patience counts for everything.
P-47N with max gas and triple tanks will give you nigh 2 hours of flight time at approximately 25-30K feet. Ta-152 with a drop tank will give you over an hour easily.
I used to fly dedicated bomber hunting missions in P-47N's. If you don't care about your K/T, you can rack up some monster numbers. Get to altitude in a position where you can defend a few strats and some strategically placed airfields. Climb away from the fight, and then hover along your patrol track at 25-30K. Open up the Clipboard map and search for Dar Bar a couple of sectors behind the fight. Since you're above 25K, the P-47 can cover a sector in 3-4 minutes at approx 400 TAS. That gives you a 150mph+ overtake speed even if you're in trail of them, meaning you will catch them from a full sector behind in 10 minutes.
I'll caveat this by saying that most people don't have the patience, at least at the time, to properly set up and defend against bombers. It takes dedicated anti-bomber support, especially if there's a furball around the field at 5,000 feet and below, and a formation scoots in at 18K. No one should ever have the expectation that anyone within the radar ring of a base is going to be able to intercept before the ordnance release point. You have to pick them up early--but if you do pick them up with a bit of altitude and speed, they are dead meat everytime, even in big formations.
One night, myself and two squaddies landed 15 kills out of a group of around a dozen formations of B-24's (36 planes or so) in a tight formation. We saw them from far away on the map, climbed to altitude in P-47D40's and had our way with them.
Unescorted bombers in AHII are simply targets.