A good post as always, thank you! I do believe that different ships' score potential has a potential cap based on their stats, but I am not certain that the Evac score itself would actually be that big of a deal during colonization phase, as the more important thing is what and how you finish your Evac, rather than how many points you got for surviving under attack etc.
I believe most successful colonies would be the ones that Evac-ed playing safe, loading up on cargo and passengers before the start of the game and making sure to evac out as fast as possible