doctorq,
I wanted to do the same thing, I also have an automatic transmission. Here are the answers to your questions to best of my knowledge and experience with my SCT Strategy Flash tuner:
(2) Yes, staying 87 octane is no problem with a tuner. Hey, one of these days you may get a wild hair and try the 93 octane tunes. Custom tunes are usually supplied with at least three different octanes. Even my SCT Strategy Flash let's me select from multiple octanes, power doesn't increase much with 87 octane, but it still decreases throttle lag.
(3) If you mean pushing down on the throttle while the vehicle is off, sorry, it's a placebo. The best way to decrease throttle lag is with a tune.
(4) I have a SCT Strategy Flash, which is a Xcal II with SCT Tunes. I can adjust shift pressure ONLY at wide open throttle. Driving around town feels stock.
Personally, I like it. My previous vehicle had a shift kit and driving around town it could get annoying. Now, when I push the throttle town hard the vehicle shifts HARD. Sort of the best of both worlds.
A custom tune may firm up the shifts at other throttle positions. Honestly, I'm so happy with this setup that I've never bothered to find out. Be sure to ask whomever you buy the tune from whether or not it firms it up at all throttle positions.
By the way, when you first install the tune you'll notice it firms up all of your shifts. That's because the transmission has to relearn your driving habits, kind of like unhooking the battery.
In my opinion, yes, it is worth purchasing the tuner. I've had a tune with no aftermarket intake and a tune with an intake (the SF let's you select the stock box or a list of tuners, very easy) Here are my reasons, please no flames!
A) I hate non-standard parts. When modifying any of my cars, even in a small way, I try to make sure that the part I'm removing can, if necessary, be replaced with a stock part. I owned a vehicle once that was heavily modified...nonstandard everything...and when it broke it was an absolute pain in the neck to locate the parts. They might be making that part now, but how about 10 or 15 years? Some of us will have these cars that long.
After reading on these boards, I've found out that the GMS sensor is not the same as stock, it needs both the jumper and the modified sensor. I'm planning to have my car for years and years to come. I know one of these days the sensor may stop working or break in some unforseen way. If the stock MAF sensor goes it doesn't matter where I am, I'll just go to the local autoparts store and buy a sensor. Easy.
B) A no tune intake will help some with throttle lag, but in order to remove as much as possible you'll need a tuner.
You need to ask yourself, "What do I find annoying about this car?". Otherwise, your modifying just for the sake of modifying it. If your answer is throttle lag, then chances are you'll end up with a tuner sooner or later to decrease it as much as possible.
C) The GMS unit won't help with your automatic transmission shift pressure or shift points! Wasn't that one of your main reasons for the modification?
I was interested in decreasing throttle lag and improving shift pressure. The vehicle was already powerful enough for me. So, I bought the SCT Strategy Flash and used the stock airbox. When I had saved up enough for an intake, I bought one of those.
I didn't need to purchase an additional tune because the SCT SF already has tunes built in for a number of different aftermarket intakes. All you do is select the intake you've installed off a list and then install the new tune. It's that easy. You don't need to get another tune, you just need to purchase an intake that's supported. It has tunes for all the popular models.
Furthermore, if you decide you want a custom tune, then the SCT SF can easily be changed, for free, to a standard SCT XCALII and then you can load a custom tune from anyone.
Another solution is to wait a while, save up some money and purchase the soon to be relased SCT Livewire, it's a new tuner. I has all of the function of the SCT Flash I described, plus you can purchase custom tunes sepeartely or already loaded. That way, you can use the canned tunes until you get a new intake. (
http://www.sctlivewire.com/)
Just so you have all of the information, the biggest criticism of the SCT SF is that it doesn't produce as much horsepower as a custom tune or mail order tune. That's absolutely true as far as I can tell. The SCT tunes are a little more conservative, less power, but little or no chance of knock, either.
If your objective is to eliminate throttle delay and change shift pressure, then it's not much of a drawback. If you're looking for the maximum performance, than a custom or mail order tune probably makes the most sense. You'll just need to purchase the intake at the same time as the tuner, instead of getting them at different times.
I hope that helps!
Auto Tranny & Tuner question... 
