The seat pan is aluminum; but, I thought the rails / adjustment mechanism was steel. I don't have access to my car to double check; but, if they are steel that is a no go for welding. Aside from that, lowering the whole seat is complicated by the motor drive for seat adjustment. If you are looking for more headroom, I think the normal solution is a thinner bottom cushion.
Aftermarket seats with a non power seat adjustment or a fixed adjustment might give you a lower seating position.