Having taken a startup from 2 to 100 a lot of the comments here resonate, but now that I'm in a company of 10,000 and trying to get things down across wide areas, I would say efficiency and communication overhead are all relative and make sure you're solving the problems in front of you and not "playing house" on problems you anticipate having down the line.

You will need more structure than you had before. For instance at 15 the idea of managers is silly, everyone still needs to be contributing individually, the only thing is that you do need to minimally subdivide work so that everyone isn't doing everything. Be wary of process for processes sake, you will start to need some but you really want to stay focused on concrete progress; is everyone doing the most important thing possible at every given moment? How fast are you shipping changes, closing sales, etc? Also make sure you have the right people, don't get starstruck by big tech vets. They have many skills that will be useful—if you are phenomenally successful—but if they don't have startup experience they likely will overengineer things by default, and a significant percentage of them can not wipe their own ass without the best-in-class tooling and infra support teams that allowed them to focus purely on one domain at BigCo. Basically you need pragmatic hustlers, veterans are good so long as they aren't cathedral or empire builders. Also watch out for weird team dynamics and nip any toxic interactions in the bud, one bad apple really can spoil the barrel; that's probably the most important thing to watch out for in the 15-100 range.