Why not do one or a few as a trial balloon, see how much they actually weigh, then you can decide whether to do the rest?
If you leave them longer your feed conversion ration will get worse, i.e. you will be paying more in feed for every extra lb they put on than you have for the meat thus far; OTOH if you want gigantic roasters that may be an ok tradeoff for you.
If you're going to do some now and leave the rest to grow larger, I think it makes most sense to do the smaller ones (which will mostly be the pullets) now, as they are just not *going* to grow as big ever, and leave the larger faster-growing cockerels to get even bigger. OTOH the larger cockerels are also probably your highest-risk birds for suddenly dying on you.
Good luck, have fun,
Pat