<tests stuff>
Ok, here's what I found, with just 200 speed bonus.
1. Fights start earlier. That's good and bad; on the good side, less waiting and more intelligence faster (fighters revealing the map, etc.). On the bad side, there isn't much time to make decisions; this change would push armchair-admiral types to pause constantly. I still think that pausing should eat a timer or something so that's not the actual style of play, but I digress.
2. Capitals and other heavies get a significant bonus, in that they're not lumbering into battle, but can actually arrive in time to do things. This is maybe the best part of the change; no more "push a capital, wait a minute or more, then deploy the mobile support". This benefits the AI quite a lot, because it usually loses its screen of Frigates before the heavies arrive, and then dies. Now the heavies are arriving during the early stages of the fight, disrupting the comfortable style of slowly whittling the enemy down.
3. Shieldless ships, simply by virtue of putting Flux loads on less often, become significantly more mobile. This almost feels like a solution to the problems with shieldless ships as a class, honestly.
4. Chases are still a major drag, when speeds are similar. This mechanic, because it's broadly equal, just means you pin them in the corner faster.
5. On the plus side, once an opponent has Flux on it, the chase is effectively over.
6. Battles felt more chaotic, because enemies would arrive swiftly before your force concentration has had a chance to kill the AI piecemeal. This generally benefited the AI, rather than the player, in a 500-FP fight where they outnumbered my fleet considerably.
7. Getting Overloaded against an opponent with low Flux loads means you're just plain dead, because you cannot flee now. 200 speed means even a Capital can (generally) catch an Overloaded Frigate. That's not necessarily a bad thing.
8. Interestingly, since missiles inherit velocity from their launching ships, using missiles to slow fleeing opponents works well.
9. Perhaps the biggest problem that was obvious is that the relatively-slow deceleration values meant that AI ships tended to over-charge a bit, especially smaller, faster ships. Frigates, and to some extent Destroyers, would need a real buff to acceleration values to not overshoot too often.
10. My AI tends to make ships back-peddle when they're getting into a good range band, and that still worked all right, largely, once the initial chaos stabilized. If Frigates and Destroyers could back-peddle better, it'd be fine.
11. This change makes friendly Hull collisions, when everybody's keeping shields down, waaaaay more lethal, and that's a problem in and of itself. I don't really like friendly Hull collisions during deployment in general; it's not like ramming in a battle, it's just random stuff that hurts your fleet.
Overall... IDK, it was interesting, but no way on 500 speed, it'd make things even more wildly chaotic. At least, not done this way, where it's tied directly to Flux loads.