The next higher upgrade to the boom has a speaker grille with an external tweeter mount. Anyone know if those grilles can be purchased separately?

No the grilles can not be purchased separately, they are made with the midrange and the tweeter mounted to them. Like mentioned by others, you don't have a lot of choices with the current situation, there are a couple others but mainly the Focal's (krx2 or kr2) or the JM speakers (wouldn't recommend them, but haven't heard their latest) if only speakers is what you are definitely going with. If you are changing more and want to keep your current amp you could go with the Boom II option, if you feel that the problems some had were taken care of with the DTII update a week or so ago. I just started installing the Boom Stage II on mine and hope to have it flashed Wednesday, to see how it is. There are a few good options I think, but really, it depends on what someone REALLY wants and what they want to keep and spend.
Me, I like my factory HU and will keep it (period for me their is no other option, but that's me), so I always change the other things, and I know my goal, and cost does not factor in a lot with me, because I have built and spent enough on stereo's to fund a few lifetimes, but that is what I enjoy, and how I spend too much money. Anyway, I will do the Stage II because I think if everything works correctly, it is a very good option, if it doesn't work correctly, I will remove it, and install another option, which for me, would be the BT setup (BT4180 and the 7.1 pros) which I have sitting in a box right next to my bike. My point is, decide what your willing to spend, where you want to end up at, and what your will to give up, this needs to be decided before you can really get a handle on anything.