Yeah, you couldn't pay me to put builds on a mech that come within 15, 20% of the CDR; that effectively limits my wattage to 75 or so, and that doesn't even include voltage drop. So probably 65 on a spectacularly good day.
But with a Sig 150, I'd feel rather comfortable going up to 150 watts. The amp drain per bat would only be 150/10.5 ~ 14.3 amps (assuming the thing shuts off when batteries are drained to 3.5 volts), well within a decent battery's 20A CDR. Of course, I don't know that I could build anything that vapes well at 150 watts, but I'd be very comfortable with the amount of current.
So, in conclusion: Mechs are fine for modest clouds if you've got a good head on your shoulders, I'd say high wattage box mod for serious clouds, any day.