Hairyhogs, why on earth do you want to eat your registered middle whites, if they are intended just for meat its a waste of your money registering them (unless you just mean birth notified).
Check with abattoirs whether they take pigs over 120-140kgs, as that is the kind of weight you would be talking about at 11months old. Boars fatten quicker than gilts, so you can get them to bacon weight quicker than gilts, but in your case you don't want this (as you wont be able to get them off). I'd say why not get a gilt GOS and a gilt Lop, that way you try one of each. My neighbours had a pair of GOS to fatten and the meat was very good, they were 6 months old boys when they went and around 90kg liveweight, they used the 1lb per month feeding diet up until 6lbs and keep it at that, they didnt turn to fat.
I've heard Magalitiza are prone to going to fat, but I that's due to breeding as they were breed for cured meats (i think).
If you got two GOS girls at 3 1/2 months you should be ok, at that age they should have already been wormed, so you may not need to worm, if they havent been wormed then you can either ask them to worm the pigs for you or worm at 6 months.
(I'm not bias towards the GOS I prefer the large blacks myself). To add of those breeds I only have practical experience of the Berkshire and the GOS, the owner of the Berkshires keeps his till 10months old.