Cobb AP OTS smooths out the throttle response like crazy. I felt like a noob when I first started driving mine and as soon as I did the OTS flash the difference in throttle response was noticeable. Not really any faster, but was easier to drive. The stock map throttle is dialed in too soon, so with minimal pedal your getting maximum thrust. Gives the illusion that the car is faster than it is. Plus if you plan on modding the Cobb is nice to have for futureproofing as well. Now that I'm making power its still the best mod for the money you can get.
Ehh, so this is my first wrx as well. Come from many manuals before, classics mostly. Last was a 2015 VW TDI though (which now feels like a dog, like holy crap).
Anyhow, the clutch in this bites much quicker, super low pedal.
Let out the clutch a little to just about right before you stall, get a feel for this by practicing without shifting. Just pushing pedal in and out, listen to motor. Apply light throttle, monitor usually tells me between 5-15% acceleration depending on grade of road.
You know the rest if you've driven a stick. As said it takes practice. Be intentionally with each shift into 1st. Try something eaqch time until you figure something you think you can replicate on flat ground. From there grades become a simple math equation of more throttle for bigger hill.
I will say I have hill hold in TWO cars now and have never found it an issue. In both my VW and the WRX, if you didn't know this, tune in, will drop hill hold when you step on the throttle. Makes it very predicatble how it will work, let clutch out a tad, throttle and let clutch out the rest of the way, perfect start.
I'm still mastering the wrx but I'm definitely getting better. Also recommend moving your seat forward, more control of your leg/foot.