IIRC, all AVM devices with working DSL are based on a Lantiq chipset with 500MHz CPU (xrx200 or vr9). See How can we make the lantiq xrx200 devices faster for a discussion on how to make them faster. As-is, they reach up to 80MBit/s or maybe a little more.
More powerful devices have a VRX518 (vr11) DSL part, paired with a more modern CPU (either Intel/Lantiq or Qualcomm) - the 7530/7520 line is one such example and probably the best supported one. There is somewhere a thread discussing the speed you can get from the 7530, but I was unable to locate it at the moment.
Btw: Used 7520 devices are very cheap, I bought one on willhaben although they are cheaper in Germany.