I have a Mark IIC+ HRG, which is the model the JP2C is based and have gotten to AB them. The JP2C definitely sounds terrific and I think blows away any of their other currently made amps. I'd say it sounds 85-90% as good as my HRG, which is VERY impressive imo. The original just sounds a little more raw and organic, bigger bottom end, it also has a sharper (sometimes piercing depending how it's dialed in) treble compared to the JP2C. Channel 2 of the JP2C also has a bit more of a upper mid presence compared to the original, while channel 3 is a little darker and more compressed, well suited for leads and single not type riffage. I also found the cleans on the original to be just a little more 3D and rich sounding (basically same differences as I found on the drive channels), but again the cleans on the JP2C were fantastic and also don't break up even with the gain turned up super high.
Obviously the JP2C kills the original feature wise, and the reverb is 1000x better on the JP2C (the original's reverb is useless) and the shred control is godsend! It does exactly what I wanted out of all the overdrive and boost pedals I've tried. It would be amazing if Mesa released a pedal that does what the shred toggle does. I'd be willing to pay a lot for that since it's so perfect at what it does!
Overall, I prefer my original, but I'd buy the JP2C if I found a very good used deal on it. It would be a great option since it's replaceable unlike the original. I'm guessing maybe the components they had back then could be why it sounds a bit more raw and rich. And mine also doesn't have the original tubes or caps, just in case someone says that's why it's better