So you've gained 1365' in 2.92 miles. You plug that into your handy/dandy incline calculator:
and you get a incline of 8.8%,(which seems awfully low for that hill)
Now, the math is the math. I'm sure the incline calculator is giving you the right % for the numbers you put in.
1365 feet elevation gain
5280 feet per mile
2.92 x 5280 = 15,417 feet
1365 / 15417 = 8.85%
As you said, the math is the math. It works out exactly right. As for 8.8% seeming low for the hill, well, that is the right percentage.
Whether you have a 32 or 34 big cog on the cassette, it won't make any difference climbing, you won't notice any difference pedaling.