My gut says its not about the the weight, but the water sloshing around. That 320lbs moving around could cause the tanks to move and cause issues with the plumbing or it be dislodged entirely and fall out (that's happened to some, but usually they are mounted poorly to start with).
The reality is that its 11 years beyond warranty so anything that happens is on you now. Since you don't need all the water is there a way to put in a valve between the two tanks and close one off? I have no idea how your pump, drain lines, etc. are set up on that model, but might be worth taking a look at.