To get them to gain weight fast to be butchered at 18 weeks or so, I'd give them lot's of high protein feed and make sure they're never close to out, and supplement with other high protein treats, such as various bugs. I once heard of a BYCer growing her own maggots to feed to the chickens. They're free and very high in protein. But if you're like me you're
right now, LOL.
Leghorns are pretty well known for being some of the most slenderest chickens around. Easter Egger roosters usually aren't all that big either. The Barred Rocks are your only dual purpose birds, but when crossed with a Leghorn... You can certainly still butcher them, but if you're familiar with dual-purpose breeds like Buff Orpingtons you're going to be disappointed. If you're only familiar with the meat chickens at the store, you might as well give the roosters away as they become apparent, as you're in for a big surprise.
To sum it all up, they'll be small and very slender and not at all feed efficient. That will be some of the most expensive meat you'll ever eat. BUT, they'll be absolutely DELICIOUS in chicken noodle soup and chicken and dumplings.
But a roast chicken? Not a chance. You'll also get a bunch of "free" pullets. But if I were in your shoes, I'd give away the roosters as soon as they become obvious (Leghorns mature very early, by 6 weeks you should have most or all of them sexed).
Just my $0.02!