I don't know for mountain gearing, but I do know on my roadies, if I don't have a 16-tooth cog, I miss it, no matter what I have in front. It's just too big of a jump between 15 and 17. So you might look at what those cogs have for their in-between gears and consider what you need there, too. It's worth it to me to sacrifice the 11 on top, to get that 16, if that's the configuration that's available.