Re: editing in 1080p without converting to optimized o...
OK, you can't measure resolution in GBs. It doesn't work like that. This technology is a huge gray area, it isn't black and white like that. Many factors come in to play. H.264 of Long-GOP, and ProRes is Intra. Apples to Onions. Don't look at file size to base resolution on.
Go to the Project Library, highlight your project, go to the Inspector, Properties, at the bottom right is a button with a wrench icon. That brings up your Project Properties.
Putting 1080 clips into a 720 project doesn't in any way decrease the amount your CPU and GPU have to work to reproduce those frames. It is still the same amount of data, PLUS the extra work of scaling them down to 720 for real time playback. Keep your project the same frame rate and frame size of the original clips. That will give you best performance.
If you shoot 720, edit 720, you'll see not much difference, but on some systems, you could, over native 1080.
Also, don't have too high of an expectation of what that rMBP will do. Its not the speed of the drives that is the final determining factor, although they play a part. It is the speed and power of your CPU, and very importantly the GPU in your computer. That is fixed, and what you have is what you have.
And even though the H.264 files are small, they take infinity more CPU and GPU power to play back, due to the way each fame must read 8-12 other frames at the same time to un-compressed, be reconstructed, and flashed on your screen.
Your rMBP is the most powerful "laptop" Mac around, but not the most powerful Mac, and not the most powerful editing machine. You'll get what you get out of it, and you will easily push its limits. But for what you'll get, it is a really great editing machine.
Also, what about backup? What happens when (not if) your LaCie or system drives crash? Do you have backup drives?