The U360's function as gamepad devices, so yes, you could do without a key encoder (eg, I-PAC), if you are satisfied with 8 buttons per stick, with no additional admin. buttons (although you could still utilise admin. functions through the "shift button" functionality of the U360).
To connect the 8 buttons to the U360, you'll need to purchase an optional wiring harness for each stick (or do your own wiring, but you may as well make your life easy and purchase the harness).
With regards to restrictors ...
If you are seriously considering a 4-8 way restrictor, then think again about whether the U360's are really best suited for your needs. The whole advantage of these sticks is their ability to map from a 2 way stick right up to a full analogue stick. But if you use that restrictor, you'll never be able to use it in that fashion, so it defeats the whole purpose of the stick.