I understand. So, the welds have not a direct effect, but have an indirect effect on the flow because it is usually seen that the welded edges have sharp corners and rough edges which causes the grass get caught. This may be avoided by making a continous good weld. So, why are some manufacturers stamping their decks if with a good weld this grass-caught-under-deck problem can be avoided? I can think of 2 reasons; Making continous good weld (and grinding the weld to make it smooth) means some extra labor cost. On the other hand, with stamped decks, you don't need to spend much manufacturing time per deck. But the deck has so big size that its initial investment (its steel tooling cost) will be very high amount that a small manufacturer can't pay. Also, the other reason is that manufacturing stamped deck will be much faster than welded decks which will cause manufacturing cost per unit deck will go lower than welded deck. I think these two reasons are main reasons why manufacturers making stamped deck. Finally, but maybe more important, it's high speed air motion through the deck channels will allow less tolerance in deck channel configuration, but with welded duck, staying in the tolerance for all decks you are producing will not be so easy, espcially if you are using older weld technologies in your factory.
By the way, another thing; why not opening some very small holes on top side of deck? I think this could balance vertical forces on the mower blade. Has anyone seen such a finish mower with holes on deck top?