After seven years on a V20, I pulled the trigger on one of the first V20c's to come out. Like you, I had fiddled quite a lot with that V20 until I got the fit right for me (at not quite 5'5", I had to figure out how to make it a shorter person's bike). But I learned a lot with all that fiddling and trial and error, which made tailoring the V20c to my size not too hard.
Still, you raise a good question about the V20c being enough of an upgrade to make the switch over a well-fitting V20 worth it. I like the thru axles, the flat mount disc brake tabs, and the carbon fiber bits make the front end feel a little more unified. But after swapping over my Dura Ace parts, the V20c was not appreciably lighter than the V20, so no real gain there. My average speeds on the V20c are a smidge higher, but that could have to do with me putting deeper wheels (36mm to 48mm) on the V20c, and my feet rising three inches because the standard "medium" 19-inch chainstay is the shortest one on offer; I had been using the "short" 16-inch chainstay available for the V20. The three inch rise did not affect handling, fortunately, and the V20c's straight slider put my hands right where the V20's curved slider had them, but I do think the longer stay put my feet in a slightly more aero position. A happy fit accident that gave me a tiny speed boost. If I had put those deeper wheels and medium chainstay/straight slider on the V20, I probably would have had a very similar small kick in speed.
Anyway, I'm glad that I went for the V20c. The updates are nice. But... After having ridden both the V20 and V20c for thousands of miles I've come to see that the V20 is still a really, really good bike. For me, the riding experience on both, with fit being roughly equal, overlaps a lot. A lot.