Get an external SSD then. If it doesnt work for you, the rMBP just isnt made for you.
A reduced portability because you carry around one additional device.
Seems to depend on the software. Swiping spaces e.g. has slightly lower FPS than usual, though the speed is the same and doesn't reduce productivity. Scrolling certain very large websites like theverge.com can happen with slightly slower FPS than usual, most of the web is fine, though. I've heard activating the dGPU eliminates those problems, though that obviously means reduced battery time when not plugged in. Doesn't work on the 13", obviously.
Ergonomics doesn't depend on the thickness.
External displays work the same. FPS reductions aren't visible on them, even if the retina screen is running at the same time. No idea why, but it's like that.
And the retina screen IS better. It's like comparing the iPad 3 to the iPad 2 or the iPhone 4 to the iPhone 3g. Once you go retina, you'll never want to go back. It's a game changer.
You can, but it's not exactly user servicable.
So you CAN put in your own SSD? That's a big bulk of the cost.