Hi John,
The heat-sealed bonds are actually stronger than the fabric itself (you can see this demonstrated in the fabric comparison video), so reinforcing the seams by sewing or adding wider strips over them would not increase the durability of the packraft overall unless you reinforce the tube fabric as well. You could do this by adding an extra layer of fabric anywhere that you are concerned will wear or tear (e.g. the floor or the lower/outer part of the tubes).
Cheers,
Matt