Kratom does reduce the effects of stimulants & the other way around. It also reduces the side effects.
I never enjoyed MPH, so I can't really speak on that particular combo. I also have a kratom tolerance, so I'm not sure what it's like for someone without tolerance.
I personally prefer to drink my kratom when coming down from uppers to reduce side effects & to be able to sleep.
I did enjoyed mixing 30-50mg of 3-FPM with ODSMT though (just boofing both).
If the opioid is dosed highly enough to cause sedation / nodding, a small upper dose can stop you from falling asleep & can sometimes increase euphoria.
I'd disencourage recreational opioid use though. A slippery slope to follow. Dependance sucks.