I agree with you- flush mounting may have some theoretical benefit, but it's probably not a big deal (probably not noticeable) with a 15" speaker. It would look better, so it may be worth doing for that. I'd say do it if you have a router and can do it, otherwise I wouldn't worry about it. I also think the 5mm flange is way too thin to mount that big of a driver to- that's only 1/5"! Brace would take care of that, though.