How would Zhang do against the prime version of these guys?

Several reasons for that:

a) He was high-risk/low-reward to start with
b) He signed with Roc Nation. Roc Nation was absolute ****ing disaster for a number of up & coming boxers.
c) He has a history of bone and joint problems (I've heard he has big calcium deposits on his heels) that led to him looking bad on occasion.
d) The Chinese weren't really seen as credible boxers

 

Meldonium itself isn't really considered to have all that much beneficial effect. But the lore is that it's a 'chaser' that can mask the presence of other PEDs in the system.

 

If prime Povetkin couldn't stop a gimped-up Jason Estrada - I wouldn't conclude he stops a healthy Zhang in 4 rounds.

Judging by his performances under adversity with Forrest and Hrgovic, Zhang might also have the biggest heart in HW boxing since Oliver McCall.

 

Povetkin had terrific movement, too. Great legs while he still had them. Best HW pressure fighter in his era.

Even so, I lean towards Zhang.

 

Jennings doesn't have much of a heart. He had a real chance to beat Wlad heading into the championship rounds but refused to take any chances. He accomplished little more than signaling Wlad was on the slide