That can be true in general, but not particulary when talking about Ampere. GDDR6X requires more power and offers higher bandwidth. But if your product isn't able utilize all the bandwidth, it just consumes more power and becomes less power-efficient. Look at RTX 3070 with GDDR6 and RTX 3080 with GDDR6X. The later has 70 %(!!!) higher theoretical bandwidth than the former, but offers only 26-32 % (1440p, 4k,
ComputerBase) higher performance. RTX 3080 has the same number of ROPs as RTX 3070, slightly lower boost clock (so fillrate is probably lower for RTX 3080) and the bandwidth cannot be efficiently utilized. But the GDDR6Xs run at full speed and consumes more power.