So far, my observations.
Level 10. Units move a lot faster. It's interesting. The big asteroid units, when killing my scouts, move about half the asteroid distance- if you add more features, you should make it so that seedlings can fight other seedlings from nearby asteroids if they happen to collide.
Growth is a lot faster, and it takes longer to take over a planet with a defence tree. Slows down the early game a bit. Def trees fire a tiny bit faster I think, and the trees grow faster.
Hm. Slight bug. You made the units be produced faster, and so because the enemy is stupid they often send small numbers of troops to your planet. Because your trees grow so fast, because units are produced so fast, it's very very hard for the stupid AI to pose a serious threat. So early game, there's less danger, less excitement. Humans are smart enough to estimate how many seedlings you need to take an enemy planet, the enemy is not.
Low energy seedlings seem to have a lower health. It makes the enemy a lot weaker, since their AI can't exploit this advantage. Their low energy seedlings die in droves. If they have the luck to be born on a high energy planet, they're effective, otherwise not. This means that for much of the game you face 0 challenge.
It's very hard to take the large planets with a high energy, since they have an extremely high health, and regeneration. Time to mass a lot of troops.
Skirmish 6. Well, no chance of taking them early. Time for mass colonisation.
Incredibly easy. I built a few dysons, ignoring their feeble assaults that didn't take into account the snappy trees. I then crushed them with ease. The extra health and regeneration effectively removes any unit attrition, if managed well, and means you end up with an unstoppable force of high energy units.
Level 25. Very easy. Now I've worked out to go for high energy ones, it's obvious strength and speed are pretty useless, and the atttrition of your forces is essentially nill. Your planets are fortresses, so most of the time you can ignore assaults or send 20 seedlings to defend. Flowers are notably more useful here, since you can easily route them to a key planet.
Eve100 is pretty hard. The lack of any low energy ones is lucky for the computer, the ai attacks a lot, and the sheer number of enemies makes it hard to win, due to the fast spawn rate. These modifications essentially make any large scale battle into a slug fest. You can both produce troops very fast, move mines very fast, and so the lines just move back and forth. Super seedlings were notably more powerful here.