RE: Foam Seawind EP from Great Planes...
your logic is good. I just went to my bench and checked to see if there was a weight on the 1500 mAh I am using. Sorry to report, there is none.
If you read my earlier post, I commented that I had the Battery all the way forward in the nose to balance the bird with the receiver and esc in the "optional" mounting location. This location is recommended as best to protect against potential water damage. the larger batteries you are considering will not slide all the way into the nose. Therefore, you will more than likely have to add additional weights to get it to balance. The Great Planes web site shows a 3200 battery. I didn't look at the weight. Seems you can add them all up and if they don't exceed the total weight with recommended motor and the 3200mAh, you will probably be ok.
gotta go get ready for work. Good luck.