Yes, parts have been a problem. At the moment, they're available again. But I say again, because SBP drums were not available new for few years, just a year or two ago. Replacing your SBP rear drums meant buying NOS at stupid prices or gambling on a used set being good enough to use. Sure, you can get them right now, but it's only a matter of time before they're unavailable again. The only thing they fit is Mopar A-bodies, and a ton of people do the BBP conversion. Demand for those parts is dropping, who knows how many more production runs will be done. You can do a search, there were plenty of threads about the only source for SBP rear drums being eBay from guys making a killing on their prices.
And, you still haven't shown me a 275/60/15 that isn't a hockey puck, a drag radial, or an autox/track tire. BFG T/A's and Cooper Cobra's are 400 tread wear, A B compound, all season hockey pucks. They can't compete with real performance tires, and they don't offer real performance tires in DOT legal street tires for a 15" rim wider than about 225.
17's ride hard huh? How come pretty much all new cars use 17 or bigger then? Or maybe the problem is your suspension is incorrectly set up for modern tires. Because usually, that's the issue. I've been running 17's or bigger for years now, no bent or broken rims. And while my car does not ride like a wallowing marshmallow like the OE suspension set up does, it's not any more harsh than most performance oriented new cars.
Found the answer you wanted to hear huh? Why bother asking?
You wanted to know if there are reasons to go from SBP to BBP, you got lots. Tire selection for 14 and 15" rims is minimal at best compared to anything larger, and you have no good performance tires available as real street tires in those sizes. Brake parts are more expensive for SBP brakes and parts have been hard to find in the past. Wheel selection is limited. Brake selection is limited, there are a few kits with SBP available but they too have their issues (larger hub diameters than OE, different lug sizes than OE, different offset, etc). And of course you can rule out any large diameter brake rotors because of rim size, but with those tires it doesn't matter anyway because you don't have the traction to make use of better brakes either.
If you want to stay SBP, knock yourself out, your car after all. But there are plenty of reasons to go BBP. If they don't apply to you because you want to run stock looking rims with hockey puck BFG T/A's and stock brakes, that's fine. But that doesn't mean there aren't still a bunch of very valid reasons to ditch the SBP.
