Hey, I really can't answer your question based on experience, but based on what I have read, and with a couple conversations with Mike B, whom I think knows best.
I have an upgraded HRG, and a SG being upgraded by Mike right now. They both have a factory RP11A circuit board which I think is the same board for factory C+ of the same config. Mike B. will tell you that an upgraded C to a C+ is exactly the same circuit and it's just as good as any other original C+ and a great sounding amp. When I spoke with Mike on the phone last august before I sent my HRG to him, he told my mine had missed being a factory C+ by a few weeks, and I responded with something like "ahhh crap!" Mike then said what am I worried about and that I should just be happy that I got one, because when he is finished with it, it will be basically a brand new amp, and as good as any other HRG C+, upgrade or not. Any major difference will be with the transformers and maybe the tone sucking GEQ. There are those that argue the GEQ hurts the tone, while at the same time IIC+ without the GEQ seem to take longer to sell. an upgrade won't have the 105 PT. most likely the 100, which is still a beast of a tranny, probably more aggressive tone. The only C+ I have experience with are mine, but from what I have read, there will be some tonal differences from amp to amp, but You might find that with any amp line.
My amp sounds killer, and I'm not about to complain about not having the 105 tranny or simul, though I'd love to hear a DRG next to mine. Someone who has more C+'s than even God, told me that one amp that he really misses was his SRGX IIC+. And he has a coli or two mixed in with an insane collection. A IIC+ is a great amp in any config, upgrade or not.
where are you in Illinois?
scott