I'll add another vote for a second antenna on the mizzen I have used an AIS with a splitter and never noticed a performance problem, but having a back up antenna is a good thing. It is the way Harmonie came to us, and we would not change it.
As for the performance of an AIS antenna at mizzen mast height, that's where ours is. We routinely see Class A AIS broadcasts from 50 nautical miles away. Class B broadcasts with their lower power are broadcast power limited, not line of sight. We expect to see a Class B transmitter by the time it is 10 miles away. Since we have a Class B transceiver, our expectation is that other vessels will not see us at a range of greater than 10 nautical miles. This is also not going to be affected by antenna height. I imagine we could get a little increase in Class B receiving and transmitting range with a higher gain antenna, but probably not doubling it.
While theoretically, an antenna at the mainmast head will increase the receiving range for a Class A AIS, in the real world increasing the range beyond 50 miles, this not likely to be of any real world use. In fact, our MFD can not effectively display AIS targets at that range.
Your instrument upgrade path will can be a fun one! We are just approaching the end of that path, and it is nice having a system that does everything we want it to do.
Fort Lauderdale, FL, USA