Never had an issue, altitude wise, when in CO or down here at the same 4900' level, when it comes to smoking meats. Start early, plan on it taking longer than usual and keep an eye on that hopper.
Time is the first thing to go out the window when it comes to smoking.
I've done numerous pork shoulders, some are done close to expected time, others are going to be tomorrow's dinner by the time it's done. The only way to get a handle on approx times is ... continue smoking meats.