This is what I mean regarding the right shift not having a linear effect:
Top Left is the gunnery table.
Top 2nd from left is the gunnery table right-shifted.
Top 3rd from left is the difference in dice between the gunnery chart and a right-shifted version. As you can see, the rightmost column is unchanged, whilst the chart really haemorrhages dice at high firepowers.
Top Right is the difference in dice if the chart is right shifted with a FP increase of 1.
The Bottom row show RS plus FP 2, 3, 4 and 5.
As you can see, the rightmost column gets steadily better as you add additional FP, representing a buff to the orks long-range firepower.
The bottom of the chart also gets steadily better; by the time you get to +2FP, FP6 and under is almost without exception improved. At RS+3, you can be pretty confident that FP13 and under probably won't be worse than without a shift.
Meanwhile, the upper end of the chart still haemorrhages dice. If we were thinking about IN Dominators, on the regular chart 2x FP12 isn't really any difference from FP24. Ork ships using the RS rules would be far better off firing individually than grouped: The hypothetical 2x FP12 or 1x FP24 does 2 or 3 times worse than if they were firing individually.
If you think about 5 FP4 escorts on RS+3, they get an additional 10 dice in the leftmost column compared to the standard chart, and lose 1 dice in comparison when firing together. That's an 11 dice incentive to only take minimally sized squadrons.
Now if you're happy with this I guess that's OK, but remembering to add an extra modifier, combined with the fact you're buffing the RH side of the chart whilst nerfing squadrons, seems like a lot of faff to me just to put a higher stat in the profile.