Precisely the above was why Artillery Overload was introduced. To penalise the players who but stupid amounts of artillery. (Takes me back to the 1/300 tabletop days, back in the 1970s when there were no army lists and 1 guy at the Nationals turned up with nothing but off-map artillery and a dozen FOO jeeps to take the objectives and spot for the guns - the next year, official army lists appeared
)
And if you
really think you need the artillery, by all means go for the overload - the penalty is not too much, so if you give away say 300 points, that's the cost of an MBT which is neither here nor there, just make sure the extra guns work hard to make up for the penalty points given away.
Planes do not count - they are limited by flight numbers.
Ammo carriers do count - they are a force multiplier that only humans can take advantage of.
As to the reduced response of D/S and G/S artillery - there is no delay if they are used as pre-programmed artillery so they are generally only useful for the attacker, as cheaper planned strikes. They fall on pre-plotted gold spots at the same delay as others, so may be useful for cheap defensive fires if you plot your FDF spots correctly.
And also, I will be looking into artillery overload for scenarios - and probably
not charging it for these since the designer knows what was needed.