Much of the reason IV heroin feels good is because when people are a little sick or at baseline when they shoot and within seconds they go from 0-60 (or even a negative number if sick). It's not only getting to 60 but that transition from sick or normal TO 60 that makes it feel that good. Similarly if people drink 2 beers an hour for 4 hours their BAC will be higher than if they slammed 4 shots in an hour but (at least for a short while) the person who slammed the shots will feel more fucked up because they got their quicker.
If you're already at '40' from the methadone then going to '60' with the heroin isn't going to feel nearly as great as getting there from baseline and in pursuing that amazing feeling, the chance of fucking up and ODing are simply not worth it. As a lot of people mentioned, you can take some time off the methadone and be able to get higher but you have no idea what your tolerance is at right now so you might underestimate your tolerance wasting your dope and end up being disappointed or shot too much and overdose.
Also, it's just such a bad idea to get back in the mindset of wanting to chase that high. So many people start popping benzos or other drugs with their methadone in pursuit of that and wind up in the ground. If you are still having issues with craving or other problems, get some help. Shooting heroin is not going to do anything but make you feel better for a few minutes and risk your life as well as the progress you have made on methadone.