MPE here. It has an exotic sound under 70% or more engine load between approximately between 3900-4900rpm. No drone.
I've heard Akra Evo in person, stationary revs. Bit too raspy for my liking. I don't think the Slip on would be that bad.
You definitely need to hear both in person. When I heard MPE/Akra Evo on youtube with relatively good speakers VS in person, it did not sound close.