I have the same setup, and it's great, but I always want to be sure people understand about the T-motors. The key benefit is that they are quiet, well-made, and permanently lubricated with ceramic bearings. But as for power and efficiency, it's really only a marginal improvement 5-10% at best. You might notice it, you might not. However they are much more efficient at turning the larger props than the stock motors are, so you get a sort of 1+1=3 synergy there with the larger CF props.
For me, the motor upgrade alone added no measurable flying time, but the CF props added perhaps 90 seconds to 2 minutes and a great deal more lift. If you use that lift to pick up another battery as the OP is doing, then I can see where 20 minutes would be possible on an otherwise unladen Phantom.