Yes shadows are an issue. That's why I have a diode on the output of each pair, to isolate them. I always have 2 pair in sun, with 2 pair on one side of the boat and 2 on the other (if there is sun ). Other than sails, there's nothing that shadows both pairs on one side. (A rigging wire shadow doesn't really show a noticeable change on the ammeter, but a mast shadow does.)

I've had it both ways and found that I got MUCH better efficiency and a longer "charge day" by running at series-parallel higher voltage. I also have fairly long runs and pretty narrow (14 AWG) wire, so that's a factor. It would be a major project to upsize the wire, and negatively affect the aesthetics too.