I like the idea. However, they'll need to be pretty good upgrades, or else players will simply ignore them as not being worth the high costs. And as far as move speed goes, I can't think of anything besides an unrealistic number that would actually make it worth spending resources on. Attack strength, attack speed, armour, HP, and EP are the prime candidates. Produce and harvest speed would go against the "less units" approach you are trying to create. Move speed isn't all that useful (sure, it's nice to reach the foe quickly, but doesn't really merit the large resources expended. Sight and attack range are a bit iffy, especially since attacking foes while remaining outside of their sight can be problematic. Attack, attack speed, and maximum EP are highly useful on the offense; while armour and max HP are very useful on the defence, making these stats the most likely for players to spend resources on, provided their costs are appropriate (no one wants to spend 500 gold for an extra 5 damage).
Of course, all factions would have to receive similar upgrades to maintain balance.