Solved

Staff of same level (and same level) as me with 4.5 and 5 stars

I don't know whether or not the devs have future plans to make staff attributes influence performance but at the moment you only need to consider stars, nothing else matters with the possible exception of age... You don't want to lock yourself into a 50 race contract with someone who is going to retire and then find it costs you a fortune to buy them out of contract when their star rating reduces as you level up.

Yes, 5 star is better than 4.5 star even if the level is the same.

I had the opposite effect with my other account once. All the staff with the same level as the team showed 4.5 stars, also the CD I hired. After I leveled, early into the season, this CD remained 4.5 stars and since all the staff of the next level again showed 4.5 stars I saw no reason to replace him (in other words I couldn't find a suitable candidate anyway). At begin of the next season the starting CPs however dropped by quite a bit, just slightly above another teams car with a 4 star CD. While only the stars should matter something is broken there. For now I trust the level more, which is the stat the stars should depend on anyway.