This is my Braemar jacket


This is one of my Argyll's


As you can see they both look different from a sport coat, they are shorter than a sport coat, and as M@A said above there is room for the sporran, also notice the difference in the sleeve detail, the pocket detail and the epaulettes.
Those are the differences and the reasons why a sport jacket does not work well with with a kilt.
I would still say see if you can rent what you need to complete the kilt outfit so that it will work for the wedding, best of luck with it.