That's not the pontoon, it's the motor pod behind the pontoon. Without the hole the open area where there motor is mounted would fill up with rain water.
I would consider sticking with a 4 cylinder in VF175 or F200. If you're going to pay the weight, size, and price penalty of the 6 cylinder then you might as well go with a 250 or 300 IMO(I understand that is not an option for your hull).
You're dealer is either uneducated, or he's lying because he has a VF150 in stock that he want to move. Either would make me consider a different dealer.