How much 'extra weight' can a P2VP carry without effecting battery life drastically?  Will using prop guards effect flying time too??

Using prop guards will not effect flytime, - my experience.

Agreed. You're going to have to strap on a lot more than that before you will start to notice a loss in flight time.

so say, 300g? 600g?

DJI states Max take off weight: Take-off Weight 1.3kg MAX
Mine carries a bit more and still flies perfectly.

My experience is that using prop guards will effect flying time if you are at or near the weight limit.    At an elevation of 5700 ft,   my phantom struggled to stay in the air with camera, gimbal and prop guards installed.    Removing the prop guards gave a little lee way.    Adding the new 2312 motors and 9450 props and new esc allowed me to start using the prop guards again.