Ultra downside would be somebody like Balentien who didn't actually catch on over here. Reasonable expectation is somebody like Trumbo or Chris Carter (or if you prefer, Dave Kingman) who will crush homeruns and strike out a lot but never really produce a lot of WAR for you. You could throw Alfonso Soriano in there too in his OF days. Upside would be a more athletic Nelson Cruz, Frank Howard or Richie Sexson type who can play the outfield full-time.
I can't bring myself to go full Albert Belle on the upside comp because it was the height of the steroid era and Belle was an absolute monster. But that sort of scary frame and ability to flat-out damage a baseball would be the target for Tank. He came in as a raw body builder and his batting eye has been getting better even as he's gone to face much more experienced pitchers at higher levels.
The problem with O'Neill's upside prediction is that most of the guys who hit like him and got really successful (Cruz, Belle, Sosa) have the PED question surrounding them. So either Tank will try to be the best of the non-roiders, or he'll need to get his comps as far as Ks and walks into a more reasonable area and get some different comps as a major leaguer.