Hey Nessex! I used to be in the same boat. A few years ago I used to keep everything MIB. Then, as you have, ran out of room. Didnt want to get rid of the collection, so to lose the packaging was the only way.
I'm not gonna lie, I didnt want to do it. I decided on a few sets that I still wanted MIB. and left those. Once I ripped open a few packages, the rest was a cakewalk. I decided that the vast majority of minimates there isnt much markup on resale and I wasnt planning on selling anyways, so that made it slightly easier.
for the most part, I unbox everything now as I get it, or have time. dont leave much MIB anymore. and I havent looked back.Actually made me feel much healthier about my collection since it didnt take up as much room.
I dont know the exact count of packages I opened...but it was enough to fill up 2 very large black trash backs.