You are right on the money about the Civic- before I bought the GTI, I test drove the SI and knew immediately that it was going to be the car to beat in G Street. My car is a Sport- the diff & big brakes are standard. I've done everything you listed except pulling the ABS fuse- I don't believe that's legal, although a blown fuse??? Will be putting on Koni Sports (still on original struts/shocks) & tweaking the alignment a bit come spring. The rear is about balanced where I like it with an 034 25.4mm RSB in the soft position. Rear tires are 215/45-17; fronts 245/40-17 on Enkei Fusion wheels. Had a ringer (ex pro rally & Can-Am) driver drive my car at the last event of the year this past fall when I was recovering from wrist surgery. (He's also a top level autocrosser.) Even he couldn't quite catch Dave in his new SI, although he got within a half second or so. He really liked the balance & performance of the GTI. Next time I see Dave, I'm asking him if we can swap & compare cars at the next test & tune. Now that would be insightful.
Here are some thoughts from 3 years of owning a 2017 Sport DSG and campaigning it in GS on and off again:
I did a full season on stock struts, a hollow 25.4mm Megan Racing rear swaybar, and Nexens. Took top pax at a local event for the first time ever and had quite a few 990+ results.
Next season I switched to Konis and the Neuspeed Race 27mm hollow rear bar. I eventually settled on running the Konis at full stiff in front and the rears are set at 2/3 stiff (since they can't easily be adjusted on the car without hacks I leave them that way). I felt the car was much faster, tighter, and neutral with this setup, but I was not able to replicate my success of the previous season for some reason, even after switching from the then-old Nexens to new RE71Rs. That season was when a few drivers locally switched to Civic Si's as well, and I was consistently unable to keep up with at least the best of them. I ran against an identical GTI at a local National Tour event and came up a little behind, so maybe I wasn't at the top of my personal best game, either. I've always ran the stock alignment.
I think with the optimal GTI you'd still have a chance, but it will definitely not be easy. Mine is 3100lbs in race trim, and I know the DSG is sometimes a hindrance for not being in the right gear (but it also helps sometimes when you can blast straight through gears 1-2-3 on fast sections).
If I were shopping for a GS car, didn't insist on hatchback and DSG practicality, and could get over the ridiculous fake vents and hideousness of them, I'd get a Civic Si.