I have a set of Imperials in one of my guitars. They are fantastic! Very open sounding and neutral, very dynamic. Amazing in every aspect ... aside from price lol. They will not be bright at all, though of course it depends on other factors such as your amp, effects, etc. I have them in an Agile AL2500 with an ash cap and maple fingerboard on mahogany body/neck. I had a pair of Bare Knuckle Black Dogs in there, but they did not suit the guitar at all. I took a chance on the Lollars and it worked perfect!
In an SG, I think that the Imperials would sound very good. A comparable pickup that I know well would be a Bare Knuckle Riff Raff in the bridge and a Stormy Monday in the neck. The BK set will have more push than the Lollars, but the Stormy Monday would definitely be sweeter sounding than the '59, if you find it to be too bright ...