It will slip. You really need to put a piece of 2 mm line inside the D12 where it is to hold in the cleat. D12 can easily be flattened in cleats and then creep. By putting a filler line in as a core you can prevent that. Putting a line inside D12 is not as hard as its sounds, but special tools really do help.

Personally I'm not hot on a 3 mm thick retrieval line. Will feel like a maffioso is cutting off your fingers with a piano string.

Wouter