When building a product with a small team, there are two overriding "gut feel" metrics that crop into my head. "Is the team flying in formation?" and "What's our average velocity?"
I don't mean that in terms of tracker tickets cleared, bugs squashed, or code commits. I'm talking in terms of achieving aggressive deadlines. Does the team consistently outperform itself? Are you getting leaner, meaner and faster every day? Can you change directions in search of openings like a world-class runningback?
Hard metrics when tracking progress are somewhat crap. Some days are bugs, some are refactoring, some are feature building, some are slog work. Any metric is going to look like a rollercoaster, so you have to go by feel. Does the pace you're setting scare the shit out of you? I'm not talking about staying up 2 days straight to cram out a feature. That's great if you need it delivered by a hard deadline, but honestly, then you just screwed up your sleep for the week and you're going to be a waste. That's a machine out of sync.l I'm talking about your average velocity over a period of time that matters to you.
Are deadlines getting beaten, or pushed back? If they're being missed, you have a problem that you need to fix. If they're being beaten, you can handle more. Crank up the throttle and make next week hell week. Dive in with some insane deadlines and let the frenzy ensue.