I have a 2011 GT that is mostly stock. I do have a tune, radials, converter and cobra springs. But to answer some of your questions.
1: You won't be able to cut a light shallow staged if you are truly reacting to the third amber. I was consistently .14-.16 unless I anticipated. You'll have to either anticipate or deep stage for decent RT's. But even still, a tune is a must to fix the throttle response. Deep staging is even difficult to cut a light with if you barely go deep turning off the top bulb. Without a tune you'll still be .06 even if deep staged. Like I said, unless you bump in past deep.
2: Weather for sure will affect these cars. But they actually can be consistently predictable. But you need data and know how to use weather stations to predict. I can actually predict within .02-.03 first time rolling in with no time trials by just using weather. Unless you have massive changes in weather though, your split times on any day should be fairly close so you can predict using weather factors.
So, yes, these cars are not dial-in machines that repeat the same numbers day after day and give you a comfortable RT staging shallow and staying 100% stock. But these cars can be very predictable and can compete with the best cars out there but they require a lot of patience, data and work to "figure" these cars out. I personally like it though.