Honestly, assuming that improved [unit] improves the selected unit by a set amount (say 4 points worth of improvement) regardless of what unit is selected, then it would be much more beneficial to select an infantry type unit anyway, since as the others stated, you generate far more of them than you would specials. In the time you would get a four point boost on a special D, you could likely make around 40+ infantry units, which would be an extra 160 points floating around on your side. Not to mention your special D would only be boosted by about 5-10% of its total stats, while the infantry would be getting a 25% or so boost. So yeah, mathematically speaking, there isn't much reason to pick a special over infantry for improved [unit] anyway. As for the improved [stat] only working on infantry, I can see that, since you get the bonus on any unit you produce, so while you're still going to get a bigger boost out of making infantry with it (both in total points and percentage wise), it isn't like you have to make a trade off when picking the boost. Still though, a +1 combat will be fairly noticeable on an infantry, but likely much less so on a Special. I don't see much reason to have it affect only infantry.
So, to sum it up: I don't think restricting improved [unit] to infantry is really needed, since they're the better bet to take it for anyway, and I don't think restricting improved [stat] to infantry is particularly hefty, as specials get such a relatively small boost out of it anyway; They'd be far better served by getting an improved storeroom and making them stuff via fabrication.