He was an excellent outside boxer with long arms. His power/strength will suffer somewhat, but he still would have a reach advantage over a lot of his opposition.
He would KICK THE **** out of anyone dumb enough to fight him from 154 to 168. Then he might take a belt at 175 just because. Marvin was the total package. Ward would be a solid match, but Hagler still beats him. I don't see him going down to 147... if he did, he would put someone in the hospital.
Hmmmmmm..... 154 vs. Floyd: Floyd WOULD NOT make the fight happen. Simple as that. Too big of a "risk". He would come up with an excuse, or try to force the fight at 147. This would never happen. If it did.... be very, very close. Hagler by SD. 160 vs GGG: Don't know enough about the hype of GGG, BUT... Hagler can either slug or outbox GGG. GGG would try to slug it out, and I can get a KO by a Hagler counter left hand. Would be a fun fight to watch. 168 vs Ward: Great fight. Ward manages to stay on his feet and Hagler takes a competitive UD. Martinez: Seriously?
Golovkin is good, but he has not earned the right to be compared with Hagler. Maybe some day, but not yet. 154/160 is Hagler territory. I'm not sure how he'd do at 168. He wasn't a big middleweight.