For 15-18% grades, I'm standing up, going about 3 mph, at one pedal stroke per second, or 30 rpm. I'm not having to push too hard on the pedals, but I do have to carefully balance the bike at that low speed.

This doesn't max out my heart rate, like I would if I was standing and doing 50-60 rpm. Sitting and mashing the pedals would make my knees sore.

My low gear is a 34-26. If I had a mountain bike with much lower gears, I'd be sitting and spinning up the hill.

So, you have a 300 to 325 foot climb in a half mile (that's 12%)? My seated climb limit is around 6%, about 325 feet per mile.