Most boats these days -- except for small daysailers -- have roller-furling genoas. Necessarily, because of the upper roller fitting and hoist, this means that the top of the genoa will be somewhat lower than it would otherwise be if the genoa were of the hank-on type. I remember when I converted from hank-on to roller furling (with a ProFurl LC-42) it was necessary to shorten the luff of the genoa somewhat.
Re: the comment on "lower than the mainsail", that's just my belief...not necessarily fact. The belief is that the portion of a genoa between its foot and the vertical distance from the foot to the height of the mainsail foot is largely "lost" in terms of increasing the "slot effect" on the mainsail. This "loss" may even be increased due to healing, as the genoa foot rolls down lower than the mainsail foot. However, the lower portion of the genoa clearly contributes to powering the boat on its own, witness the huge deck-sweeping genoas on racing boats.
Just a notion. I've not seen any "proof" of this.