As these are the number of promotions in Spatzimaus' mod, I assume it's balanced
Not quite; I also decreased the XP per fight an air unit gets from 4 to 2. After all, barring those few turns when the units are healing or rebasing, bombers are going to be getting in one fight per turn for pretty much their entire lifespans. So I needed to slow down how quickly they were gaining promotions.
But the general sentiment is okay; starting an air unit with 1 promotion isn't unbalancing; it's just when you can get to 2 or more promotions easily that there's a problem. 10 XP is fine, 30 is problematic.
Secondary note: in my mod, when a unit is created it gets the Rookie promotion. For its first fight, that unit is -25% to combat, but gets double the normal XP from that first fight. After that fight the Rookie status goes away and the unit progresses as normal. So that doubled XP from the first fight goes a ways towards a new promotion, meaning an extra ~5 XP for land units once they've had a fight.
Tertiary note: I've also got it giving certain "elite" units an extra +10 to +30 XP when the game starts, regardless of what buildings are in the city. That way I can have a few units within a given combat class starting with more XP than others. Only problem is that while you get the XP at creation, you can't use it until the following turn or until you've taken certain actions.
Oh, and something I spotted a page back about the Imperial Road Network suggestion:
Movement bonuses are not found in the Buildings table and I can't import them from somewhere else. I'm not sure if your proposal can be done at all with currently available tools.
They're not directly in that table, but you can do the next best thing. In my mod, I've got a wonder called the Planetary Transit System that, among other things, gives every friendly unit that started the turn within my borders a temporary +1 movement promotion. It's a simple end-of-turn Lua check, looping over every unit to decide whether or not it should have that promotion. It's not an explicit road boost, but it has a similar effect.
Alternately, you can create a new hidden and Disabled technology. If a civ builds this wonder, use a Lua event to give it that technology at the end of the turn. Then, in the CIV5Routes.xml table, you just add a movement boost on roads/railroads for anyone who has that technology, in the same way that civs get road boosts currently at Machinery. The math is a bit complex, but it shouldn't be too hard to ballpark it.
Since it's a Disabled tech, no one would be able to get it through Great Scientists or Research Agreements. However, city-states and barbarians WOULD get it eventually, through their secondary "tech stealing" mechanism (which ignores the Disabled flag), and if you use any other mods that depend on the number of techs a civ has (like Tech Diffusion) it'd count towards those.
Basically, if you're willing to use a little Lua, there are several ways to do things like this. In my own mod I've got wonders that give all enemy units within my borders a negative promotion, buildings that give units that start the turn in that city a temporary promotion (the Space Elevator and Aerospace Complex, which give paradrop-like promotions to mimic the old Airlift mechanisms), a wonder that gives all enemy civs -10 Happiness, and so on. So a lot of things are possible.
and try to combine it with some others ... the various building effects and wonders and technology etc... to me the changes make more sense than the vanilla versions.(well at least for me 
)
Got it now, great tip, at the perfect time!
and +15% wonder 
