D3 cannot handle the power that a type-r can. yes the SQ of the d3 would be better, but powerwise the d6 and type-r are a better comparison. SQ of the diamonds would be better than the alpine but i still think that you can run more power to the type-r than a d3.
D6 has an "over" sized 3" coil. I've got 1200 on each on each 12 they don't even get warm. The D3 has a 3" coil as well. The D3s are good for 600-800 rms. When I had my D3s they got roughly 600rms. In my personal opinion the diamonds will get louder off less power. I think the diamonds can go in a smaller enclosure too. I'm just more prone to diamond lol