Just played arena 7 a few times, and my immediate impression is that the new defaults go a bit far with the mine speed. I thought mines did move too slowly before, but now because they can reach and orbit an enemy asteroid very quickly, a mine is almost guaranteed to wipe out any flowers there, and that is far too imbalanced IMO. On the one hand, if you don't have a mine, you're totally screwed if one gets to your asteroids early in the game. On the other, erasing flowers means that the mines themselves will be wiped out very quickly and thus the game will rapidly tilt in favor of whoever owns the last upgraded defense trees that survive the great purge. I would say, yes make the mines move faster than before, but only a little. Also, I think by far the most influential factor in determining the strategic impact of mines is the frequency with which flowers are generated. Any change to mine attributes will lead designers to rebalance the flower generation probability so I think it's a net wash.
I do however like that flowers move quickly, since that lets you send them where they will be useful instead of "wasting them" on a small planet only because it would take too long to reach your powerhouses. There are still a few outstanding bugs in how flowers are handled however, and if those could be addressed, it would help to make flowers more logical and consistent in how they affect strategy (upgraded defense trees on on a captured asteroid no longer produce mines if the mine is destroyed after the asteroid changes ownership; the AI will plant multiple flowers on one tree).
I don't know if I like the idea to make weak seedlings weaker and strong ones stronger, either. If anything, I would have voted for narrowing that gap. There is something very satisfying to me with the "ants devouring a caterpillar" strategy if you can pull it off. For Arena 7 in particular, trying to win with a weak starting asteroid surrounded by strong ones was a nice challenge. It was difficult, not guaranteed, and took a lot of strategic thinking. With the new default settings, it seems you need slightly larger forces in general to take an asteroid, and doing so with weak armies is now much harder. Seedling economy is still the most important element in Eufloria, so even with the change, whoever has the most seedlings will win.
I also like the tree roots planting faster, but maybe you could slow that down a little. The length of the root path is a meaningful random factor that can have a big influence in the early game as it affects how quickly you can get your asteroids pumping out seedlings.