Just wondering if any of the common coilover kits (especially JIC) use 2-1/2" springs where it would be cheap to experiment with rates using widely available and cheap Eibach ERS springs?

JIC, Cusco, Ohlins, Zeal, HKS, ............ basically all of them use the 2.5" id springs (not all the same length though)

as the other person said......most JDM coilover use 2.5" (65mm ID) spring. When you replace the spring with ERS just make sure you buy the spring with same "Free Length" (or at least as close as possible).

fyi: 1inch = 25.4mm.