Smart move on leaving the extra room. Once you have them trimmed down like you do, you can now tape the 2 halves together and figure out the fit, then trim what you need.
I also have slightly larger biceps and forearms, so my cover strips were 20 mm I believe. 15 mm is pretty thin when you actually look at it. The issue that caught me was the space between the cover strip and edge of the forearm was too wide when I applied for certification and I had to trim a bit down. so I would lean to widening the cover strip to better cover the raised flat area.
Some of the more experienced builders can please correct me, but from what I understand, the 15 mm is screen accurate but if you need larger, it will not affect your approval as long as it looks proportional. The thing to aim for however is that the width of the cover strip on the forearm and biceps is the same to give the uniform appearance running down the length of the arm (same for shins and thighs).