The problem as I'm seeing it on PC is that if you select a shop item that is "new", meaning you don't already have a version, then you can hit the key (or R2 on controller) to bring up the stats of your currently equipped item from the same slot. This is perfect.
The difficulty is that if you select a shop item that you already own, whether it's equipped or not, then it shows you the item's upgrade stats if you buy it. But there's no way to compare that to your currently equipped item.
So if I'm wearing light armor that's as upgraded as I've been able to make it, but I have other light armor in my inventory, then selecting the "other" one in a shop shows me how good it will be upgraded, but I have no way to know if that would be better than what I'm already wearing.
So as mentioned, I have to exit from the shop completely, open inventory and look at the stats of what I'm wearing, and then go back into the shop to see if it's worth spending the gold to upgrade the other item.
And if there's 2-3 items at one vendor that will upgrade stuff in my inventory, I have to do this multiple times. It's really inconvenient and breaks the otherwise good flow of shopping.