Hey Dan, I have a cumberland which has foam seat and back. The weight of the foam is negligable and I found that after about 4 hours of fishing the seat is still ok to sit in. The back of the seat is a slightly different matter.
I weigh just under 350lb and have found that the back of the seat tends to give a bit as it warms up, to remedy this I have placed a piece of HDPE 6.5mm plastic in the back of the seat to stiffen it up. I have a bulging disc in my back and found the Cumberland seat ok apart from the movement in the seat back after prolonged use.
There has been a comment made by another user of a Cumberland FT, relating to the back of the seat wearing/softening with time and extended use, but then most things wear with time.
I have yet to test the seat with the HDPE backing, but I believe it should perform better with the added piece, giving better support.
Although added expense, the foam in the seats is replaceable should it be damaged and probably with a bit of experimentation the right type of foam could be substituted to get the required firmness. After looking at the foam back It would be possible to glue a minimal foam lumbar support into the seat back if required, as there is enough room/movement in the fabric back to do so.
My size, is the main reason that I have found problems with the seat back but from reports by others, they are more than pleased with the performance of their Cumberlands and do not appear to have any problems with extended use.
Matter of choice I suppose and style of tube you go with. I offer these comments as constructive critisism of the Cumberland seat and my experience with it.
I am more than happy with the tube overall and its performance.