The thing I find with using the BCP just for idleing up is that it will rev up way higher than 1200 on start up. I have a volt meter wired in my gauges, and there's sure not much more than 12 V there for the first minute or so after start up. If you run BCP right away, the idle tends to get up to 1400+ RPMs pretty quickly. I use the PTO switch, and it keeps it at 1200. But is the torque converter locking up counter productive when you are trying to let the truck warm up?
I use the BCP all the time, works wonders trying to boost dead batteries, especially on powerstrokes, tractors, and irrigation motors.