It's more flow restriction on the air side. On the whole, you stand to gain more in terms of power, efficiency, and drivability from the individual cylinder control of port injection than the loss from air flow restriction. That's on a boosted engine, of course. I wouldn't presume that to be the case on NA engines. That said, I have seen plenty of secondary "shower head" injectors on motorcycle engines. Those are in the air flow path and could be considered a restriction.