I dunno, you have a pretty good kit for what you are doing -- a very good camera + low ISO values + smallish prints = good quality. The 5D was and is a remarkable camera, and you really aren't testing its limits. The newer cameras will give you a couple of things:
Live View. This is pretty handy for nailing the focus. Using an LVD magnifier like the Hoodman loupe and zooming in with the LCD zoom (not your lens) results in very accurate manual focus direct from the sensor itself.
Better high ISO quality. But you don't really need that (although you might discover a penchant for hand held dusk photography with a 35mm f/1.4 lens. That sort of thing is a lot of fun.)
Bigger prints or more cropping room. But again, unless you want to buy a 17 or 24 inch printer, this isn't a huge advantage.
And none of these things are likely to improve your images so much that a non-photographer would be able to see the improvement in 13x19 inch prints. In 16x24, yes, and in 24x36, definitely.
The lenses are another story, and something like the Canon 24mm TSE lens is just so superb that it's likely to make you happy even if your friends don't see a huge difference. (And then you'll want the live view option for critical focusing, since that lens is manual focus only.) The Zeiss lenses appear to be the same way, online tests show very significant improvements in edge sharpness in particular. Not sure how visible it would be in a 13x19 print, however. Others will likely weigh in here.
One of my students just brought in her new 6D for me to check out. It's a very nice little camera...