Downside to a lot of offset cookers is their lack of airtightness. You mentioned heat loss with this but more importantly, air getting into the firebox except through your controlled vent is bad. Depending on the size of your smoker, but for pretty much any backyard smoker, I like to light about 12 kingsford charcoal pieces in my chimney. Once they are good and ashy, I put them on the pile of unlit charcoal, then shut up the firebox. You might want to put some of the higher temperature aluminum foil insulators style tape over firebox openings once you have it all shut up with smoking wood in there. This will help keep it airtight during the cook. It is better to bring the smoker up to temperature, than try to cool it down, so don't overshoot, and be patient. When you make an intake adjustment, give it time to take effect. Once you have some lit charcoal in a pretty much airtight firebox, the BBQ Guru should be able to do its thing and regulate the temperature. It simply controls the only air into the fire.
Traeger Texas, UDSx2, Weber 22.5" OTS, Weber 26.5" OTG, Weber Performer Platinum Blue