I feel like I should post an update to this.
First of all I had the same experience as
@xavierny25 had. My water pump went "rocks in a cup" like 2 weeks after I put on the 2325 belt. The dots didn't connect in my brain at that time but I'm positive the longer belt contributed to it. Second, it's not true that the belts are "basically the same length", as I wrote earlier. It was an error on my part, I just didn't put enough effort into measuring them properly. Sorry for that. Even after all those years, 2320 is still measurably shorter. Third, I got much more vibrations in gas pedal, belt squeal when cold and general lack of smoothness when running the 2325 belt. A few weeks ago I bit the bullet and paid the shipping on a new 2320 belt from the US and installed it. The outcome of all this for me is the following:
1. If your truck runs a 2320 belt and you need a new one
DO NOT even think about buying a 2325, thinking 5 mm doesn't matter.
2. If all you ever knew was a 2325 belt and your truck runs OK then... well... it's weird but good for you
Now I'm just wondering why all that confusion about belt lengths. When I checked Rockauto a bit deeper I found out that it does indeed prescribe a 2320 belt for the 9-7X (among all the 2325 belts) but only starting with MY 2008. Did GM change pulley size on any of the belt accessories or did they start using weaker tensioners at that time? Or is it just that they finally acknowledged that 2325 belts are too long and lead to parts loss and changed the spec quietly?