This could work depending on how the magic damage calculation is actually done. I don't know if nexon does something weird with INT and/or magic attack in the calculation. Depending on that this could be a great buff or a big nerf.
I personally like this idea. Maybe not make LUK a 1:1 ratio but if LUK was worth more damage wise i would definitely add LUK mid to late game, which would give mages are similar path as thieves, statwise.
Why stop at extra slots only for mage equips? Why not give all classes extra slots? Yes, it's a big buff for all classes but Horntail is not gonna die on its own 😛