Hi Kyle! One thing that would help is to know exactly which version of the P1 you have. If you've bought it in the past 6 months it's probably a 1.1.1, but it'll be good to confirm that (it's printed on the sticker on the box and the one on the bottom of the Phantom itself).
To be honest, adding the tmotors or any other model that fits on a Phantom isn't really going to net you any significant increase in flight time...but that doesn't mean there's no reason to do it either. I have the tmotors on mine right now and while there is a slight bump in efficiency it really hasn't translated into a noticeable increase in how long I can stay in the air.
You best bets are to focus on weight (as you are obviously aware of), and components that can augment your flight time, such as the 9443 blades that come stock on a P2, as well as a bigger battery (2700-2800mAh).
Note: if your Phantom is a 1.1.1 then you should already have the motors with the CW+CCW threads which will allow you to use the self-tightening P2 props. However, if you have an older version without the cross-threaded motors, DJI does make a version of that same prop that isn't self-tightening. The easy way to tell is to check the prop nuts that came with your Phantom...if they're all the same color then your motors aren't cross threaded, but if two are silver and two are black then you are all set for the self-tightening version of the props.