Unless this is the most poorly managed system ever (cringe), they should already have plenty of storage area for this.
I don't think doubling the inventory space is unreasonable. There is simply not enough space to store alternate mod sets for characters and keep all those potentially good or great mods that need sliced.
But... I guess it's a somewhat hidden way for them to monetize simple inventory space isn't it? Gotta buy your way to quicker slicing. The slicing mat income isn't enough to keep up a good turnover pace on mods. I'm sure some people will save everything and always bump the cap no matter what it is but I don't think the cap is reasonable right now.