On photozone, they gave it "3 to 3.5" stars out of 5, that's not a low score. It is above average. And yes, they do factor border performance into the score which is likely why it didn't go even higher despite the good centre performance. My opinion is generally to avoid looking to closely at overall single value scores, since unless you value the factors that go into it exactly the same way they do, it will vary from your needs. Look at the detail and decide if the lens is the one for the job. Also note their scale is absolute not relative. A good lens for its type can still score low. e.g. even expensive zoom lenses can struggle against budget primes in score.
Forgot to say, they also have a "field quality" score which tries to look at it from the view how it would meet the needs of how it will be used, and there it scores 4 out of 5 stars for sports and wildlife usage.
