It is thread jacking, yes 
Find MapSizeDefs in the data/game folder, then change every instance of <MaxHabitablePlanets></MaxHabitablePlanets> to 999999. This should remove the arbitrary cap and you should get more planets.
On the big maps there is no way you can move from your starting world to your empire's edge at even 40 speed. By turn 200 or so your empire is about 100 moves across. Simply trying to move from one end of an immense map to the other at 20 moves per turn takes way too long to do. This is fine for medium sized maps in my opinion but not for the really big ones.
I don't really agree, and there's a couple of reasons why:
First up, we have the simple fact that every point of movement is better for the player than the AI, due to the AI only ever attacking once per turn with any given fleet. This alone is, in my opinion, enough to justify capping speed somewhere around 20, simply to prevent the player jumping into a staging area, wiping out 5-6 fleets, and then fleeing to safety. Until the AI can do that too, very high movement rates are simply a buff to the player.
But above and beyond that, being able to strategically redeploy your whole military from one end of the empire to the other in a matter of 5 turns makes it near impossible to be caught off-guard. At 40 moves per turn, I can almost certainly bring my entire fleet to bear on any point in an astonishingly short amount of time. There's no line of supply; and no risk of strategic over-extension. There's no reason to use the movement-increasing modules on MSBs and fleet units. Making the player too mobile is almost as bad as making him not mobile enough.
Now, what I'd propose is this:
1) We base engines around a decently-armed ship having a top speed of about 20 moves per turn. Note that there's no hard cap - but going over this will require using more than 50% of hull space as engines, and so will be severely detrimental to the ship's armament and defenses. If you want a rapid-response unit, then by all means design them with 70, 80% engine; but you will not be able to go toe-to-toe with a fully-armed normal ship.
2) I buff the MSB movement boosters to halve the move price of tiles within AOE. This means that, provided you spend some effort on the infrastructure, you can rapidly redeploy within your own territory, but will not have this bonus map-wide.
3) I buff the fleet movement-booster on ships to give +50% movement, but make it massive (like 1 per ship with engines and not much else). It will be vulnerable, particularly to enemy missile fire.
If you want to zip from one end of the galaxy to the other, then that's fine - but you're not doing it inherently with a huge warship loaded with weapons. That's poor design, as it cuts back on strategy.
