Found this in their FAQ:
" How much "weight" can Magnetacks hold?
The "weight" a Magnetack can hold is highly dependent on the thickness of the item to be held. Magnets lose their strength the further away they are from their mate. It is hard to put numbers on this so we've broken it down into examples:
Unframed photos of various sizes. From A10 all the way up to A0 size.
Paper-thin items: a single Magnetack can hold up to 20 sheets of copy paper or newspaper pages, or several sheets of photo paper.
Laminated items: a single Magnetack can hold up to 10 laminated sheets of paper (depending on laminate thickness).
A standard unframed poster (24" x 36") can be supported by one Magnetack in each of the top two corners. However, we recommend one Magnetack in all four corners to prevent the poster from swaying as people walk by or by A/C.
For items slightly thicker (poster board) or larger size posters (72" x 48"), it may take several Magnetacks placed along the top to hang those up.
Framed items - unfortunately Magnetacks are not strong enough to hang approximately 99.9% of all framed work. No, we haven't actually done the math but if you were able to use Magnetacks to hang something framed, please let us know!"
A board and sleeve may work with enough magnets, but i wouldn't trust it with anything valuable? might have to try this sometime.