They say there is a big jump in sensitivity between the IMX and a GLX. I've fished an IMX but not a GLX. I have a new NRX in the garage that I haven't used yet. I'm sure the St Croix guys probably could relate it to their line of rods too. From what I understand at a certain point there is a diminished return in performance on the high end rods. I'll be honest the only reason I bought a NRX is because I got for a little over the IMX price.
Maybe this will help clarify things to, I live in Michigan so many of my bottom contact rods are spinning rods. Most of moving baits I fish with baitcast rods but in my mind I don't need a sensitive rod for that. More for fishing a tube, drop shot, twin tail grub. Does that make sense? Why would I need a $300 baitcast reel to pull a spinnerbait?