Well with the science one you do get more than just 'goon' boosts. For instance you can help research vaccines that are ailing a race. Other than that I think you're just better off gifting the stations to someone because they're pretty worthless.
p.s. If you want to capture stations super fast just go auto combat. I had the entire belt of stations and it only took me 6 turns every time. Sure it's cheap but it beats going through the tedious motions every time you want to take one over.

All outposts have diminished returns, how much diminished idk, but generally you wont get much from > 3.

Did you outsource them prior to the construction? Because AFAIK outsourcing doesn't appear to actually change the build time for buildings you initiate; however, if you check the race's "construction quality" ratings (e.g. on the planet info screen), you should (hopefully) find that they've gone up after you outsource one.

I'm pretty sure that if you don't outsource them then you get the benefit of reduced construction time.

Or maybe it's the other way around - I mostly tend to just capture them when the opportunity presents itself, then use the outsourcing/gifting as a way to curry favour and/or help out a particular race.