about the distance, i don't think it affects performance but thats just my opinion
if you don't want the galaxies to cause lag, try deleting the galaxy, turning the
dust multiplier down to something like 0.2, and recreating it.
sure, it might looks less pretty, but it'll make your sim run faster. Even a couple milky ways at 1x dust multiplier doesn't cause me (and hopefully you) any lag.
Dust particles have no 3d'ness to them so it takes several thousand of them to start lagging at all. Objects need to be rendered in 3d, so anything more than 500 or so lags me. It doesn't sound like that is the case though, are you running any cpu intensive programs while having using ubox? anti-virus scanning? when i close them out, the fps generally goes way up.
If your system gets past 500 planets, moons, and stars that'd be pretty impressive! (and i want a copy of the sim
)
Also do you have preformance issues atm? or do you expect them to happen as you go along? Anything below 10 fps starts hindering me imo. Turn off trails and labels if you're not using them too, it halves my lag frequently