I've read that Preston has a carry weight of 165lbs. If I equip him with modified items (such as pocketed armour pieces), will it increase his carry capacity?
According to this wiki page (toward the bottom of the page) companions are affected by pocketed armor.
It is possible to make companions wear pocketed armor to increase their max weight (+30 pounds with basic pockets). There is also a skill magazine perk that will increase the companion carry weight by 10.
Actually I think SPECIAL attributes remain unaffected. I equipped Cait with the Grognak costume, which is supposed to increase STRENGTH by 2. But her Carry weight did not increase. Don't know about pocketed armor, though.
Edit: As other have pointed out in the meantime, it actually does affect carry weight! But one has to get out of the trade-menu and re-enter for the effect to kick in. My bad!