Using a 4 slot double growth weapon/armour gives no benefit over a 8 slot normal growth weapon or armour if you're going to level up more 8 materias in total. You'll get them to master twice as fast, sure, but then you need to repeat the process for the next four materias. The total time spent would be the same as for an 8s 2s growth equipment.

The only piece of equipment that actually gives you more total AP than a 8s 1x growth equip, is Cloud's Apocalypse, which has 3 slots and 3x growth. Cid's Scimitar has two slots and 3x growth, but this does in fact not gain you more total AP than a 6 slot normal growth weapon.