Personally, I'd be inclined to make ART a tad more powerful but more expensive to produce (e.g., require two tungsten or something).
Another idea, that sort of works with yours, is that instead of making artillery more powerful and more expensive, give the Signals and Recon company both a 10% buff to artillery (20% total between them) in 1936 and increase the buff with each research level. I suggest this, because the ability to lob large explosives ten miles down range is a huge combat multiplier. Even the Germans did not bother with much improvements in their artillery, they focused on making the tubes they had respond quicker and accurately, which was a function of recon and signals. The Americans, with their overwhelming material advantage, built a signals and recon (forward observers) system that could perform time on target barrages with mixed batteries across multiple divisions called in from a two man patrol far ahead of the front line.
For game purposes, the Signals company could increase divisional organization by a substantial amount and accelerate planning and decelerate its loss when using the battle planner. Signals could also give a 10% artillery buff in 1936 that increases with research levels. Recon could also give 10% artillery buff in 1936, increasing with research, on top of its 10% speed buff, that could increase with research.
Making both Signals and Recon have buffs like this would make it expensive and complicated to get the best out of artillery, as it was during the war. The big boost to artillery effectiveness has always been spotting the enemy faster and landing shells even faster. Recon was what got the artillery on target, but it was Signals that let Recon call in more artillery and faster. Each new tech level of Signals and Recon would cost more in IC, men, and equipment to represent that these support units are getting larger with more capabilities.
To encourage Signals and Recon company development and reflect reality, the last two techs for Signals and Recon could also give a buff to CAS. The last year or two of the war, saw some ground units talking directly to CAS overhead and directing their runs.