It is true that DTX is reducing the accuracy of the measurements, but not as badly as you described.
If DTX had such an impact, you would see an increase in interference HO even without AMR.
Indeed, for non AMR calls, I guess the DTX is enabled in your network, isn’t it ? And yet, you don’t see high interference HO for non AMR calls 🙂
I’ll try to see if there is any history about such misbehaviour, I’ll let you know. Until then, you should probably avoid DTX for AMR calls. THe problem appears with UL DTX or DL DTX ?