Yes, it's definitely closed...as in door slammed shut!
I find the result is greater hip and should turn than with a neutral stance since my hips and shoulders start out facing away from the net to a much greater degree. In fact, I don't have a choice but to rotate shoulders and hips to contact.
When I've tried a neutral stance I find less power given my high friction, earth bound status.
@ Chas Tennis...
I developed my current stance and motion working with a pro. We tried to compensate for lack of leg drive power with more rotation (closed stance) and lack of low friction leg drive with the step into the shot football-style. The key difference is that I do keep my shoulders open to my swing path to avoid pinching my shoulder. Thus I try to swing my arm by rotating my hips then torso instead of swing the frame with my arm and shoulder.
We have lots of guys in our club who smack the ball REALLY hard with a nearly vertical arm bent at the shoulder. Many suffer shoulder pain after 2-3 sets and three are sidelined with shoulder injuries. They generate impresive pace at a great cost to their bodies.
It's more difficult building a serve based on proper core rotation but it feels so effortless when it works. My wife has complained about my pace when we "hit for fun" but from my end I don't feel like I'm "hitting hard". I feel quite the opposite: very relaxed and swinging easy.