Nahh man, the 5102 is much better then the 5100. The 5100 was litterly the first DOCSIS 2.0 modem Motorola made. Fallows by the 5101, 5102,5120,6120,5101U,6180,6580 I think is the line. Technically there wrong in saying the 6120 is not compatible. All DOCSIS is backward compatible.<Edit out a whole bunch of stuff here on how and why> Basically they probably don't have a way to capp the bandwidth, which actually happened here in alot of areas when DOCSIS 3.0 first rolled out. Thats why the 6120 became so popular. You could pay for the lowest plan, get the 6120, and get simular performance of the higher plans. The ISP smarted up soon enough though. So now they either don't allow the 6120 unless you have the plan, or they implemented a method to cap it.
But yea, 6120 is great, but you did very well to upgrade to the 5102, if for no better reason you should (I hope) be able to see the signal levels and logs now. Sorry we put you through so much red tape.