Don't expect a maintenance dose of Methadone to have any kind of meaningful effect on your day to day mood or how you feel physically. The idea is that once you're on that stable dose, it won't cause sedation or other effects one might desire from Opioids, so no, I don't think in this situation it will help in a noticeable way with your detox.
Why are you being detoxed? Is it a choice or did your prescriber tell you that you must taper? There is a big difference here. If you are tapering of your own free will and would like to go back to your 15mg dose, then why not do it? One thing you have to keep in mind about tapers, is that there will be some degree of discomfort as an inevitability. The process will most likely not be enjoyable. If you would truly like to be off of Benzodiazepines for yourself, you have to steel yourself physically and mentally for what's ahead.