Nothing like a devils advocate to help us find the right answer! I'm no expert either....maybe the two of us will figure this stuff out. Thanks for your ideas and input.
I think you are probably right about the tension not being that big of a deal if you are willing to go with a more slack rig than a lot of pro riggers (like B. Toss -- Rigger's Apprentice book) would recommend. And, as it states in the manual that comes with the Loos Tension Gauge, a lot of people do sail with more slack in their rig than is recommended for fear of "breaking something."
However, they also state in that manual in the "additional information" section that:
"Contrary to popular thought, a slack rig is more punishing on a hull than a properly adjusted rig. Insufficient tension will reduce the loads transmitted to the hull. Slack rigging will punish the spar and rigging needlessly by allowing excessive movement, chafe and shock loading. Modern fiberglass hulls should not be damaged by properly adjusted, tight rig. (then some other stuff here, followed by) For the properly tuned rig the leeward shrouds will not go slack under normal sailing conditions. The lateral stiffness of the mast and the fore and aft stiffness of the spreaders is reduced by a factor of 2 when the leeward shrouds go slack. This important structural characteristic is not generally recognized."
They got my attention with this stuff. To wrap up, you are right about how all the spreaders seem to attach to the hull in the same way structurally. And it would be ideal to have all the same size wires and terminals for simplicity. I think the only question is whether the mast and deck can take a little extra load, if you do want to tighten the 3/16" lowers to "proper tension" which is about 10.6%. I think the designer might say that the boat and mast could take a little extra tension and it would be OK, but I just don't know, yet -- These are some pretty ridgid little boas. They may have just used the 5/32" lower shrouds mainly for cost savings.
A little off the topic of this specific shroud debate (for those who are reading this and wonder about the tensioning of the other wires) the manual with the Loos gauge gave some pretty good info on this subject and was backed up by the info in the Toss book. The 3/16" headstay is tensioned to about 16% of its breaking strength (750 lbs) and the backstay is tensioned so that the mast is straight with the headstay tensioned as mentioned (it will be a lower tension because of the larger angle in which it leaves the mast). Then the shrouds are tensioned at 10.6% of their breaking strength (500 lbs for 3/16" wire and 350 lbs for 5/32" wire). That's it.