I strongly suspect that the feed-conversion advantage of ISA Browns is only noticeable in a commercial setting (i.e, it is probably some tiny percent, relevant only if you have a hundred thousand birds in the barn).
Any good line (if you can call it that) of sexlinks should be just about as good. (Meaning no disrespect to ISA Browns -- I have one, originally it was 3 before two expired, rather young, of internal laying type issues; I just love 'em to pieces in terms of laying ability and personality, although I am unimpressed with their longevity, alas).
I've been asking around about leghorns myself lately, and almost everyone with standard leghorns, regardless of variety or source, seems to find them to be wingnuts. Most consistant exception seems to be people with show lines of bantam leghorns. There may indeed be relatively tamer lines of standard leghorns out there, but it is not like "only the commercial lines are nuts".
I'd think for urban backyarders, a good line of sexlink (or maybe two if you want red and black so people can have Variety) would be pretty much exactly what you're looking for.
Good luck,
Pat