Ellis was a former MW who had come up short vs. name fighters like Holly Mims (he also beat Mims), Henry Hank, Rubin Carter, Don Fullmer, and George Benton. He lost close decisions to these men. Carter dropped him and beat him the most decisively. Ellis moved up in weight and did much better. He went 12-0 (6) from 1965-1968 including 2-0 in WBA HW World Title Fights. He was part of a tournament to determine the WBA HW Champ after Ali was stripped of the title. In the tournament he defeated Leotis Martin KO9, Oscar Bonavena W12, and Jerry Quarry W15. He made one successful defense vs. Floyd Patterson W15. After Patterson he did not fight for 16 months. His next fight was vs. NYSAC HW World Champ Joe Frazier to determine the Undisputed HW Champ. Frazier stopped Ellis in 5 rounds. They fought again 5 years later and Frazier stopped Ellis in 9 rounds. Ellis also fought Ali (among others) and was stopped in 12 rounds. Ellis did beat Ali (Clay at the time) as an amateur. My question is this... how well would a '65 -'68 version of Ellis do vs. the best Cruiserweights or smallish HWs in recent years? Ellis at 6'1" with a 76" reach, and 195Lbs. would not be undersized at CW. Ellis vs. Haye, Adamek, Byrd, Toney, RJJ, Bell, Mormeck, others?
Ellis would be perfect for the cruiserweight division. It is a shame for him it wasn't around when he was active, he probably would have been a dominant champ there. I think he would be competitive with all the guys you mention and probably get wins over most of them. Toney and Jones, at their best, I am pretty sure would defeat him.
Ellis is a very underrated boxer. He had excellent skills and fought in one of the most competitive eras in HW history. And came up from MW to do it. That's remarkable .