With the 2 speed; yes. Having two gears will work the motor less. You should use an XL for a single speed just because of the greater start power needed.
Yeah, I was concerned about that too. The 2nd gear clutch only engages the gear on one spot and that spot is a measly pin embedded in plastic. I wasn't sure if it can handle the snap that BL power can produce.
That's for this reason Brian that i've thin to use the Robinson hardened gear set, the pin for the second speed is plant right into the steel. Much more solid, no??
My friend had a bl revo running a 8l on 3s and the 2 speed. It ran fine, so the 12l on 4s should be ok. Go up to the xl if you want to go single speed.