Calling it makrut seems entirely reasonable, honestly. I had it as kaffir lime leaves here entirely honestly only because the recipe I was working from had it as such. It was marked simply "lime leaves" at my market, ha.

As to your question, in fact, while I haven't made it personally the original recipe I was working from calls for 1 tsp. finely chopped red chilli. I think it would take a little chilli quite well! Were I not cooking for someone currently incapable of handling any manner of heat-spice I would have added a 1/2 tsp. of chilli oil, I think, and adjusted from there to taste.