I have found the HD800S to be a much superior headphone compared to the HD800C, even when the original has the mod. The mod addresses the treble peak, but doesn't fix the weak bass or the tendency to treble harshness and sibilance. I have found the HD800S does all this. Overall a much better headphone and I would advise any potential HD800C purchasers to save their ducats and go for the HD800S if they possibly can.
The only downside, apart from price, is that the soundstage is not quite as precise as the HD800C. Sources in the HD800S have a placement that is a little fuzzy and without as much air around them as the HD800C. For me this was a small price to pay for all the other goodies that the HD800S delivers. YMMV.