But with this catch.....he must drain himself down to 160 and he must weigh 160 in the ring on fight night. No rehydration to 175. And he must be a career light heavyweight. For example, Michael Spinks draining to 160 and entering the ring at 160.
What you’re asking is impossible to answer. If this person is a career light heavy who never fought below 175 ( the way that men like Roy Jones and Ezzard Charles once did ) then you have no way of knowing how his body would react to it.
Considering a Welterweight was sent in & beat him, it could certainly be done. The caveat here complicates things, though, as it’s no longer really a Light-Heavy, is it (though I suppose you could say Leonard was no longer a Welter if he came up to beat Hagler).
Are we talking about natural LHWs? If so, none, because I don't think there's any guys that are the elite of the elite that actually started at LHW that would be able to boil down to MW.