I have a 1440 mv jon boat and have been tossing the same idea around for years. The topic has also come up here a couple ofo times.
Basically, there are two trains of thought out there:
1. Go for it, it won't make a much of a difference to your flotation and won;t have a big impact on structural stability.
2. Be careful, cause it will effect your flotation & structural stability!
I guess it really depends on the boat & how it is designed. Most bench seats appear to add a level of structural rigidity to the hull and contain foam to achieve level floatation required by the USCG. If you remove the foam, you may no longer have level floatation if you get swamped. Also, because of the loss of the structural nature of the seat, the hull may flex more than it should, causing stress & shortening the life of the hull. Adding a floor may help add the rigidity back, not sure.
You could contact a dealer and ask them if removing the seat will cause problems but, given liability, they will probably tell you dont do it. Also, if you modify the boat, and then sell it, make sure to tell the buyer!
In the end, i've never taken mine out. Since my boy will be starting to hunt soon, my approach is to convince the wife we need a new large, safer, open boat!