Actually, the "firepower" column is not part of the gunnery chart, it is merely a label. So the furthest left you could go is the "defences" column. Tables have label columns and rows which do not make up part of the sample space and are not relevant, since it's an abstraction. For example, suppose that instead of being firepower 6, 12, 20, etc, it was 60, 120 and 200 but that the number of dice you actually got to roll stayed the same. So 60 WBs would give 5, 4, 3, 2 or 1 attack dice, depending on aspect and modifiers, just like 6 WB does now. Now suppose you were firing from close range at a defence. Would you get a left column shift and go from 5 dice to 60? No, of course not. The amount of firepower is an abstraction and not a valid part of the table. This can be confirmed by the bold line separating the firepower column from the defences column.
However, having said all that, I do believe that it's "doable". The abstraction used is linear to the firepower, so instead of increasing by a factor of 10 as given in my example, it flows on quite neatly (6 WB could give 1, 2, 3, 4 or 5 dice normally, so the next logical step up would be 6). Also, this takes some of the abstraction out of the number of WBs a given ship has since it would mean that at maximum potential you actually get 1 attack per point of firepower. This makes more sense (thematically) than always getting some attenuation. I also believe that the extra column gives greater depth to the game, because it makes it more rewarding when you manage it.
So, in principle and desire I think that we should use that column. I just don't think we do, given the way the table is set out.