The issue of increasing power is sort of confusing at the moment.
A single Mk. V fleet ship is indeed 5x the utility of a single Mk. I fleet ship (of the same type, of course)
However, a cap of Mk. V fleet ships is only 2.5x the utility of a cap of Mk. I fleet ships (again, of the same type), due to the reduced cap of Mk. V ships (assuming Keith's guess of .5 cap multiplier for Mk. V fleet ships is correct)
Mark 5 ships have at least 5 times more health and 5 times more firepower, and sometimes other benefits (reclaiming better, more range, more armor, more armor piercing). If anything, with the same cap, they are at least 25 times more powerful and cost only 5 times more ressources.
As far as balance goes, I think the recap in the reference says it all anyway.
Fighter 1 against fighters 1 : draw.
Fighter 1 against fighters 2 : win 69%.
Fighter 1 against fighters 3 : win 81%.
Fighter 1 against fighters 4 : win 86%.
Fighter 1 against fighters 5 : win 84%.
If level 2 fighters had twice the utility, they should leave about half of the level 2 cap alive when fighting them. They leave 69% of the reduced cap, leading to something like a 3.2 ratio. With an equal cap (which keith can probably generate with slight modifications to the code) the percentage will probably go up to 75%, leading to 4 times more usefulness.
IMO, level 5 deserve a boost, as they should be more useful than the level 4 cap. That's about it.