I've noticed a similar problem on my new . Anybody else have this problem? I just noticed a couple weeks ago when I was getting away from the pier with strong on-setting winds. Needed all the angle I could get. I didn't measure the angles, but everything is "as-built." The steering linkage is going stop-to-stop, so seems like something more significant (like design problem). I have about a 7-8 degree difference between hard starboard and hard port.
Because of the construction and layout of the steering mechanism on an “M”, it’s normal to have a lock to lock asymmetry by an amount about what you are describing.
From memory, starboard lock is limited by one factor, and port lock is limited by a second factor. When you do get a chance to study the linkage, that will become readily apparent- they're out in the open, just pull the motor from side to side. There are a lot of variables on engine mounts etc, so your exact results may vary.
Adding guide bushings at the point where the steering arm passes through the well wall will also change this.
This asymmetry is nothing to be concerned about, really. There are lots of other factors in play here that contribute more to steering asymmetry than the steering travel range, such as prop walk, whether or not you are moving forward or backing, power level and application time, daggerboard extension, and operator skill. Be aware that, when under power, the Mac is no different than other outboard boats which steer more easily in one direction than the other. And have fun!