Nice car but you don't want to be lowering the front only. a) it won't look right and b) it won't handle right.
Thing is...the car was not meant to have 19" wheels, but that said, the overall diamater should be the same, just that the 19" tires will have a lower profile. Are you running spacers too, maybe that's why the potential clearance issue. Maybe the offsets are not correct for your car.
Long story short - you can lower it just like any 996, but not if you try to undo the good work the engineers at Porsche have done for the sake of bling.
Here's mine on 18" sport classic II wheels. No spacers, Bilstien PSS9 coilovers and H&R sway bars. It handles very well and no clearance issues. My buddy runs the same setup with spacers by the way.
