Hi Will 2010'
I'm not really knowledgeable when it comes to the technicalities of pruning, but we've got two very large magnolias that we pruned that way; had many branches that size and larger when we bought our home. My husband cut them off, and we haven't had any adverse effects that we can see. I'm no expert but hope this helps.....[the trees look nice!]

M.grandiflora limbs naturally sweep to the ground; that's the way God intended.
Limbed-up specimens are a tip-off to me that a Yankee has been whacking on 'em. LOL
It's less offensive, IMO, if you start with a small specimen and 'train' it into your desired lollipop shape. It's a sacrilege, and sadly in bad taste, but it's your tree, and you can do with it as you like.
3 to 6-inch pruning wounds will take years to heal - if ever - and in the interim, will provide entryways for rot and decay organisms to infect the heartwood.

DO NOT, DO NOT, prune Magnolias!!!!!!! They are suppose to sag that is what makes them so beautiful!