I guess based on the fact that the side panel fits back on, and the top two fans don't have much of an impact on airflow; Then the D15's only downfall (which isn't a big one, and also it's moot) would be that it's a bit uncomfortable to install; that may not even be the case either.
If I had a do-over, I would likely just go with the D15, however, I don't feel like buying one now is really worth it. Like previously mentioned, it doesn't make much sense to me to buy a new cooler, to see small to no noticeable increase in games. Plus I like the aesthetic, and ease of installation on the smaller coolers, so that too is a plus.