Even an eight minute fill, with flying time to suitable infrastructure plus taxi time etc is going to limit the work load each plane can manage although actual capacity per trip is considerably enhanced.
The benefit of the old amphibians is that pick up takes about 90 seconds and can be as close as the nearest suitable lake.
Clearly what we need is a Guppy 747 with the fuselage up side down and a water landing hull adapter bolted on the bottom. Scoop 24,000 gals in 90 secs. Now that would be some water bomber. Or maybe set the DC10 on a couple of ginormous floats?
Where have all the guys with imagination gone?