My girlfriend is on 60mg of methadone daily, 5mg of valium once daily, and 2mg of lunesta at night. She doesnt experience any amplification of effects. At night she does not have any "labored breathing" issues and sleeps soundly through the night. I did notice she is more likely to appear "forced asleep" as i like to put it, where she will make a small noise and twitch like her body would have normally woken up briefly but didnt.
Otherwise shes been on that exact combo for about 3 months now with no issues. The clinic knows about her intake and said if it were any higher she would get booted from the program. Mind you before we got on methadone she was on like 20mg of valium up to twice a day and this whole big thing lead to her coming way down before she was even thinking about getting on methadone.
(DISCLAIMER)So before anyone thinks i am saying the clinic allows for benzo use, her history at the time showed a significant drop in dosage prior to joining the program which made her look serious about quitting and trying to make it into the program. That too was hard as when we were down there she was sick and the first thing they said was "shes not getting on the program on benzos!"