I typically wouldn't use 600V class on this type of application. In theory they would work fine, however any kind of neutral voltage rise starts to push the insulation limits pretty quickly.
15 kV CTs run about $500. Since they're being used to protect 138 kV reactors and other equipment, it seems like a no-brain-er to me.
Also, 600V CTs are typically window-type, although wound-type are available in certain designs, however, wound-type are generally not rated for outdoor installation. Depending on the ratio you need, window-type CTs probably won't work. Generally I see ratios of 5:5A, 10:5A, etc... used on neutral applications. You can't get those in a window CT.