To the question of 158 grain ammo in the MP5. I wrote the MP5 Family Operator Manual for HK. At that time HK GmbH recommended anything from 115 - 147 grain ammo.
I do not recall the 158's being mentioned but I see no reason why they would not work well in the MP5. With subsonic the heavier the better IMO. One way to know if to fire some an d look for any impact (witness marks) in the rubber buffer in the retractable stock. That will indicate overfunction caused by the ammo impulse. Try to avoid that.

The issue with TZZ was not its bullet weight but the chamber pressure and more importantly the recoil impulse it generated. Now there are many TZZ loads in 9mm, but some (115 grain) was loaded to blow the nearly 1 pound UZI bolt back reliably. When fired in the MP5 it broke bolt heads prematurely. HK improved the bolt head (and locking roller holder) to address this for the SEAL's who were shooting TZZ in training and breaking MP5 bolt heads and M9 slides on a regular basis. I think we gave tem 1000 improved bolts FOC. It improved the service life but still broke bolts. Later the SEAL's dropped that round. THAT TZZ AMMO is what HK waves people off of. I would not use that is any firearm except what it was designed for (the open bolt UZI).