I really don't experience hardly any blocking effects from 30mg daily at all, even with the build up of methadone concentration over 6 days or so. I also dose my methadone in two doses majority of the days; 20mg worth of pills crushed, dissolved in H20, filtered, then insufflated in the morning, with a 10mg dose prepared the same fashion probably 8-10 hours later. I can dose a couple bags of heroin (2 to 3 decent BTH bags) and feel pretty solid, with ~4 bags giving me a solid nod with that slight difference in experience than ECP (not a bad thing, just an interesting/kinda cool thing). I'd have to dose a little more H while not on methadone, and when dose with the methadone, the experience seems to have more sedation relative to euphoria (there still is some) with the increase in overall opioid effects. There is a solid mood lift not seen with most tradition opiates, but similar to Tramadol or Buprenrophine that has its advantages over that typical opioid euphoria. The duration with heroin being the most prominent array of precieved effects is a little less than with out the methadone, but the methadones effects becoming the more prominent ones experienced, the less of a desire to redose, as well as desire to not reach as high of wonked out state like when taking most short acting opioids (if this is your thing, you just want to get high as possible and nod your balls off on short acting opioids). Methadone helps you use H and make it through to the next day before the craving of a short acting opioid become relatively intense (I'm not on MTT doses, I use methadone for neurological problems).
If you don't have that high of an opioid tolerance, and don't have to much experience with higher doses of methadone (greater than 50mg, with usual doses 40mg and below if using solo), then a small dose of methadone will help decrease the required additional opioid to achieve similar effects, atleast in my experience. I have even recommended to take 7.5-10mg of methadone with 15mg of oxycodone to get a similar experience as if one was to take 35-40mg of oxycodone. This was for an individual who never has dosed higher than 5mg of methadone and never has taken larger doses of oxycodone at once than 50mg insufflated.
Some people even say low doses of suboxone can synergize with other opioids (1-2mg or less. More than two a lot of people begin to experience the start of a blockade.
I think what would mainly make a difference in effects experienced has to do with tolerance for both drugs but especially methadone, and how often you take methadone.