For me, I can't detect what I would call an appreciable difference between how long cars look on 24", 26", and even 28" curves. When you are in that range, all you are looking for is improved tracking at higher speeds. From 22" up to 28", don't expect to get a "gee whiz" feeling when you compare them...you'll find the difference appreciable at around 30". Then, if you make the mistake of trying, just for kicks, those cars on 40" curves, you'll be disappointed with the 30" curves. The reason is that they are all just too darned tight to look right.
But, what're ya gonna do!? You have to have some curves, and they have to fit! So, find something as wide as possible that you can live with and not decide you hate what it has done to your track plan. Below 30" you can just be glad they look okay but run really well at speed. I am fortunate to have two end curves on my central pit folded loop that are near 45" radius, and they look quite good. The rest of my industrial tracks are at 24" minimums, very tight for Walthers heavyweights.
-Crandell