I have system panniers also.. however they don't live on the bike. I only use them for touring etc.
When I was looking for a top-box, I wanted one that can (and now does) live on the bike.. And went for the silver Gobi. Main reason for that is as its silver it won't get HOT in the sun.. I couldn't care less that it doesn't match the panniers to be quite honest.
And my reasoning has proved correct... in blazing sun whereas the panniers get VERY hot. the top box doesn't.. it gets warm... the inside remains cool. Which is exactly what I want.