The experience model for HOI does appear to be more simplistic than I would prefer. Leader experience is a general proxy for overall unit experience as well as leadership skills. I would like to see a more accurate modeling in a future HOI2.
Unit Experience is important and does impact Grand Strategy. The North African campaign in 1943 was waged largely to give the green American troops some combat experience and test their capabilities before an invasion of Western Europe. Experience is really measured on a parabolic curve. The first three months of fighting make a BIG difference in unit performance. The next three months help a good bit, but not as much as the first three. After a year, another 3 months doesn't help very much. After 3 years, it makes almost no difference at all.
I would like to see a model whereby units have experience and they progress in capability on a parabolic curve. Furthermore, when the units are reinforced, their experience decreases. With minor reinforcements, it should have a minimal impact as the veteran NCOs can place replacements in less critical roles until they become more acclimated. As the losses mount and there are fewer experienced troops and NCOs, the impact of replacements would become more significant. So you would have an inverse parabolic curve for experience loss during replenishment. Less than 30% losses might have a minimal impact. More than 50% would decimate the experience base.
In fact, I would suggest that experience is largely the application of the tactics gained through infantry/armored research. Units should not automatically apply the full benefit of those avenues of research. Instead, the existance of those tactics should provide some base capability (due to training) and then experience growth should be capped based upon research level, simulating the improved execution of those tactics. But in that way, the green troops of a tactically superior nation are not automatically more adept than very experienced troops of a less developed foe.
The creation of elite units could then be handled by giving some experience to the unit at creation. The builder is simply taking more time to train the unit in those tactics. Because no training can be a substitute for real combat, only a minimal level of experience could be provided. Perhaps an amount equal to the first 3 months of fighting described above might be appropriate? And that additional training, in addition to having some additional cost, would also delay the introduction of the unit by 6 months as it takes longer to gain experience through training than it does through trials of fire.
Finally, I wouold like to see experience gain be based upon the difficulty of the battle fought. You don't learn a lot wiping out a nearly defenseless foe or retreating at the first sign of the enemy. Hard fought close battles are much harder teachers. If the "odds" of the battle are 2:1 or 1:2, experience awarded should be half as much if it were 1:1. Likewise 3:1 or 4:1 should award a third or a quarter respectively.
As for playability, there needs to be some grouping and labeling of experience levels just to make the system understandable to the general gamer. Perhaps units progress from Green to Tried, Seasoned, Verteran and finally Hardened with color codes for each status to make the whole system more easily represented.
All of that is in regards to unit experience. I would also retain leader experience, but I would personally make it a bit more complex than the current system. My ideas for that will wait for later as I've rambled on enough for now. But to get back to the original point, I do think experience could be expanded to more accurately reflect both elite units and to better simulate the grand strategy moves that are made in relation to the experience level of the army.
Instead unit effectiveness depends on 1 and 2. I kinda think it's silly that green soldiers under a great general perform better in combat than do hardened veterans under an average general. But, hey should be a good game nonetheless! 
