My understanding is for steel start with 1A per 0.001" (~40A/mm) and for stainless to start with 10% less. For your 2mm stainless sheet that would be about 71A. If 40A takes forever to get going, it's still putting heat into the base metal during that time. If you can keep up with adding filler metal at 71A, your heat input should be noticeably less than it is at 60A, which should give you a better color in your welds.
