When you are at 1500psi on your hp80, how does your air supply compare to your buddy at 1500psi on his aluminum 80? (You have less air than him).
There's an important concept. Off the Florida Coast, boat operators often say "head back to the anchor line and go up when you reach 1,000 pounds" (or some similar number). That's a habit from when 95% of the divers they saw had an aluminum 80 that filled to 3,000psi.
With a 80 cu. footer that fills to 3,500, 1,000psi represents a different volume of air than the "80" that fills at 2640psi. So that 1,000psi "rule of thumb" is handy, but not exact.
We consider our tanks and plan our own dive accordingly.