In my group we have a kodiak 12x12 canvas and the big heater buddy works just fine in those cold mountain nights. No issues at all,just use a fan if you can, the heater buddy fan goes thru the batteries too fast.
Are you planning on running the heater in the vestibule? I have found that hillebergs with the 4season inner tents stay pretty warm. Expecially w 2 peeps. Gotta wipe the condensation down w a sponge though. Running the heater in the ventilated vestibule might be a great way to heat up and Zapp condensation.
I don't usually have a heater buddy w me while I'm backpacking w a hille though.