The buckle will slow you down, as has been proven in multiple studies, one of which you can find here: http://link.springer.com/article/10.1007/s12283-010-0037-0/fulltext.html
The differences are likely to be in the order of seconds. Which in a race is something well worth considering, given that you are going to buy shoes anyway, buying some that do not come with a built in aero penalty is essentially a free gain!
Might not concern you, or most CC users, but it would certainly be something I'd consider!
Skimmed it - surprised at the result of aero covers being so negative!