I must not have been clear. The MacroMate and the SubSee are external diopters that go on the outside of your housing. Results using either with a 100/105 macro lens are to my eye quite nice. The 2 stop loss that I was talking about is from a 2x teleconverter that goes between your lens and your camera. I often use a 1.4 teleconverter but have never found a 2x tele that is usable with a 100 macro that I really like. You might take a look through the pinned topic about getting more than 1:1 magnification for some other ideas.
Thanks for the quick reply! You have cleared this up quite well for me. I am leaning toward the external solution, rather than the teleconverter option primarily for the flexibility in my shooting options while diving as opposed to being "set" with what I will be shooting once I am in.
you have been very helpful; thanks again!!!