The difference between a frequency focus and spending an item slot on more uses is this:
1 frequency focus adds 1 use on ALL of your items
1 additional enchanted item slot adds 2 uses to 1 item
If you have lots of enchanted items, a frequency focus will give you a lot more than adding enchanted item slots to an item.
Sponsored magic is a kind of evocation and thaumaturgy and works pretty much exactly like that. Additionally, you can call upon your sponsor to help you, taking up casting stress for you, increasing your output, etc. On the other hand, your magic will always be colored by your sponsor, and you will not be able to do magic that is not within your sponsors domain. He might even stop you, if you go against his agenda (this usually works with compels). To stay in Mrmdubois example of Hephaestos, I could see a sponsor like that stopping you from attacking an opponent that is using an extraordinary weapon, because you could destroy the masterpiece. Or to make you repair any damage that you (or even others) have done, because he is a constructor, not a destroyer. It gives great opportunities for roleplay.