Unfortunately it is generally not a good look to heavily focus your cover letter on one area of the firms expertise. It is a very good way of showing the firm that you have done lots of research and are genuinely passionate about the work the firm does, but can sometimes have the effect of demonstrating that you aren't fit for the TC as during the TC you have to do all of the seats in your rotation, not just the one you are adamant you want to do. Not to mention, when it comes to retention, you may not even get the chance to qualify into your first-choice department. It runs the risk of GR discarding you on the basis that you aren't very open minded to working in other practice areas, despite the fact this may be a gross false assumption. You're much better off using your main interest as a springboard for why you applied, then expand your desire to join the firm for its breadth of practice areas, rather than latch onto a specific practice area. I've made the same mistake previously as I have quite a bit of sector specific experience so that's just my two cents, take it with a bit of salt.