Ryan, as a life-long Catholic (currently lapsed), it's been my experience (at least locally) that Saturday Masses are much more casual than Sunday morning - especially the High Masses (Easter, Christmas, etc.). However, the general trend is that Sundays are also becoming more & more casual.

That said, I think your outfit will be just fine. I'd suggest just a small bit of color (maybe a red pocket square?) as an accent.

Welcome to the Church!

Regarding the thread as a whole, I agree that trying too hard to 'fit in' makes one's appearance in a kilt look more like a costume than 'normal' apparel. Trying to 'out-Scot the Scots', as it were.