I have been taking weight out of cars and off bicycles for 46 years now and have been working on the ST a great deal as do not even drive it yet.
1)Battery and mounts, 36lbs, you only need one cable to the rear but it will probably be 4 gauge and very heavy. Lithium batteries are incredibly light but also damn expensive. I am making my own mount way back and low, as close to the firewall as possible, dimple die stamped, it will weigh around 1 lb, 2 max, bolted to the frame(I might weld it in instead) I also removed the cable plastic housings in the area and will tape the wires and use a bit of loom over them, good 3M pro grade tape is all I use on a car.
Battery Mart, 11 lb, looks like the Braille, $74 shipped. (for the ground use a 4 gauge and bolt it well to the chassis in the back of the car)
Note: I have not done the calculations, weight measurements, etc but I may still put the battery in the rear as I have taken out a great deal of weight back there, 125lbs so far and need to move more weight back)
2) Windshield washer tank and pump, 8.4 lbs, less than I though but significant as pretty far ahead of the axle line and on drivers side. Unplug the line to the rear and line to the windshield by the hood hinge, plug windshield and rear line together, tuck into a gap(you could cut a bit off but if ever want to put it back you have to get more line and couplers, it would save a few oz to cut it short though)
I am using a 3 pint low weight and cost overflow tank and a pump I already have, it will be in the tire well then just add some line to the one that goes to the rear wiper to get it to the tank. I removed the rear wiper as well, 3 lbs saved so only need a single outlet pump, I do have to run a wire back to the pump to control it from the proper switch. I could easily use the stock pump that has two outlets and have a rear washer easily.
3) Negative battery cable mount on strut tower, 4 oz. I used a punch to marke the spot and absolutely must be done then a spot weld bit from HF to drill out the two welds, caution must be used, it will get away and make a mess if not careful and do not go to deep. I will use one of the engine mount bolts, sanded off paint, to tie the cable to.
4) Fog lights, just 8.4 oz each, removed as using the openings for brake duct hoses when I put on the BBK and work out the best way to direct air into the center of the rotors.
5) Steel support in front of radiator, the mount for the crash sensors, very poor design that blocks and causes turbulence into the radiator. I will remove it and make one out of round material, wing shape would be better if I could find it, top and bottom both the same shape or actually a wing to add a tiny bit of downforce. Weight, not known yet but it will help keep the engine cooler on track and save a bit of weight.
6) Stock wheels and tires long gone, street set is 38lbs per corner, race set will be 33 lbs per corners, saving considerable weight which is a highly critical area.
7) Brakes, 15" wheel fitment BBK kit in progress, not sure weight, might be 10lbs plus per corner
8) Intake, very seriously considering the cowl intake in development, stock is only around 6lbs so would save around 3 lbs but much further back, just a bit higher, still it will help.
9) Engine cover, not much weight but useless for performance, to bad the engine is so ugly though, I would live with an ugly engine over added useless weight.
10) Bigger Intercooler, would love to not add weight but no choice and with a GT25 turbo, 20-30 minute track days, going to need a fairly good sized unit, not sure on weight gain yet.
11)There are studs to be shortened, brakets to be removed or trimmed, all over the car but not 100lbs worth, not on this car, I have removed around 50 lbs on a car doing this and dead end wiring, this car might add up to only 10-15lbs from what I have seen so far.
12) Sunroof, I would not have one, period, but had to as we have two big dogs that just need the air, simple as that.
13) Spare tire, jack, covers, rear seat, belts, 125lbs off! I am making a new floor cover from Alumalite, it will weigh very little, and sound deadening but very low weight, cargo/dog divider will be a strong light weight net.
AND more as I dig in more