I personally would be pretty unhappy with a low gear of 46x17 or 18 for any significant climbs. Especially when talking about 15% grades... I'd have to walk. Back in the days when the Tour de France was ridden on flip-flop bikes, gearing for mountain stages would typically be a 24 cog on one side and a 20 cog on the other, with a chainring of 42, 43, or 44 teeth.
I live in the mountains, and I'm not in Tour de France shape, so my flip-flop bike uses a 40 ring with a 24 tooth freewheel on one side and a 15-18-21 freewheel on the other. This only works because the dropouts are a bit more than twice as long as average, and angled so that the brake pads hit the rim regardless of where the hub is positioned. Without that option, I simply would not be using any kind of a SS/flip-flop bike here.