So to sum up the current firmware options (only listing non-beta, currently downloadable):
Merlin 270: 5GHz works, Xbox doesn't
Stock 272: 5GHz works, Xbox doesn't
Stock 370: 5GHz doesn't work, Xbox works
Merlin 372: 5GHz works, Xbox doesn't (?)
I haven't tested all of these myself, so please correct me if I'm mistaken. As far as I can tell, Merlin 372 does have the Optimized for Xbox setting, but it does not actually fix the Xbox problems (probably because the older wireless driver does not support it?)
Thanks for confirming. I wasn't sure if the option they change in the driver was available in the old driver or not.
Since there are more general 5 GHz users than XBox 360 users, I will stick with this older driver for now, until the time Asus and Broadcom can come up with a fixed driver (it's coming, they just don't have any ETA about this at this time). XBox users who badly needs the option can still run 354.29.