Magnesium? Do you mean asymetric pole or symetric?
Symetric: Other than the expense (more loa means more marina fees or more expensive telescopic setup). you can get a test certificate in ORC that will tell you the answer. Otherwise i think longer means deeper = better if your boat planes)
Asymetric - as Gary says, the disadvantages outside the rating system probably make the rating question moot.
Gybe your current spinnaker in a breeze then decide if you want to reach further out to get the brace in the jaws. You will have seen photos of race bowmen shimmying out on the pole.
your mast will have a track that only goes so high on which the pole is clipped. At some point the mast-end will be as high as it goes and your pole-end will be where it is.
Then there's sail set. Your pole will probably be sized for the mast height and boat size. So if you increased your pole length you will need bigger kites cut, and they will have a less vertical luff, remembering that the overall shape of the luff (sideways from top to bottom) affects the shape of the belly.
Then there's what happens when you go too deep and the pole and kite come round to windward.. a longer pole would make this worse. Google Farr 40 chinese gybe. Spoiler alert, it was the sheet-mans fault for losing concentration and dumping the sheet.
Both these videos below are excellent.
I ran the kite on a mates boat in a coastal race recently. Spent the whole day downwind tuning the kite. As much as it would have been nice to go a fraction deeper, or fly the jib, when we gybed it in 15 knots I wasn't wishing for a longer pole.
