Dave,
The class rules have changed to allow 3/16" shrouds, so you are still class legal, no worries there.
One bonus of the 3/16" wire is that there is a handle kit from Ronstan that fits right over the turnbuckle and allows you to adjust your shrouds without tools. Unfortunately, the handle kits don't fit the 5/32" shrouds, so adjusting the inners will still be a pain.
The increased diameter of the upper shrouds means that they will stretch less when loaded up. The theory, according to some, is that this will prevent the mast from inverting and failing. Some others, me amongst them, don't agree with this theory, but being able to use the handles on the larger turnbuckles is definitely an advantage.
Why anyone would go with different size wire on the inners is beyond me. Even if you forgo the handles and use tools to adjust your shrouds, you will still need different size wrenches for the different shrouds. If I were getting a new mast, I would ensure that the builder supplied it with the same size wire for all the standing rigging. We are not a set and forget type of boat. I adjust my rig if the wind speed changes by 2-3 knots. I've been in several regattas where I adjusted the rig tension between each race...