Bill, dammit... I started answering your question many hours ago. Didn't finish, had to come back to it. Now I'm done and you've edited out your question(s). I'm still gonna post my answers!
Go with what you feel, Bill. You're on your "training tires". Do you feel like you've mastered them? Are you dancing with them right on the edge of all that they have to offer? Or close enough to that? If so, then, yeah... move on up!
How far up you move depends on your budget and on how competitive you are. The big dog RE-71's for instance... nothing else touches them, but they're expensive and they wear out really fast. If you don't "need" that competitive edge, you can get "almost as much tire" for less money and/or in a compound that will last significantly longer. (that's what this thread is about)
Load range equates to a maximum tire load.
Look it up. Figure 3000 pound car, 60% front bias, your heaviest corner should be about 900 pounds. Your 91 load index is rated to 1356, which gives you some reserve capacity to carry heavy things (or people) in your car. That's all about sustained load carrying capacity, not peak loads as you might see when a tire is loaded in a bump or when cornering. You probably don't want a load rating that is a LOT less than the factory spec, but unless you're fully loading the back of your wagon with heavy stuff regularly, you could probably get away with a LITTLE less without issue.
The S in your "91S" is the speed rating. That has more to do with how hot tire will get after driving at sustained high speed. We're not too worried about that for autocross, but a higher speed rating usually also equates to stiffer sidewalls, and a more "performance" oriented tire. (if the sidewalls flex too much, they generate heat at speed and eventually blow out... so softer sidewall tires will have a lower speed rating)
You don't "need" to upgrade anything else. You've got a fairly new car, it can cope with whatever tire you throw at it. A grippier and more responsive tire will make it possible for you to "toss" the car harder, and with soft stock springs and shocks, you'll blow through the suspension travel quicker and get onto the bump stops. But, you should already be doing that even with lesser tires. You're driving on the bump stops with any modern car in an autocross.
What you might consider upgrading first is the shocks, and adding a rear swaybar. But, I'd do tires and alignment first.
Then you need to consider what class you want to play in. Personally, I'd try to stay in S5 as long as you can. Run those 195's, and do Stock-approved suspension mods. (which, for FAST, means pretty much anything that doesn't change the springs or ride height)
But, if you want to do more than that, you could move to M4... and spend more money on more suspension mods and wider wheels and tires.
Oh, and read this:
http://drivefast.org/novice-tips/suspension-tips/
).
