I already filled the trunk with an oontz pump trunk floor replacement since Kia cheaped out on my spare tire by not providing it, plus I drive my kids around in it so a battery in the passenger compartment may not be wise. If this was a pure track car yep, it'd probably be in the right corner and the whole interior would be in the bin, the latter I did with my 82 Rabbit.Have you thought about relocating the battery to the trunk? There's plenty of room under the cover and taking a 40lbs battery from the front on a front heavy car and putting it in the rear preferably toward the passenger side would balance the car out a bit. Little subtle things like that can really help. I remember 20 or more years ago, a friend of mine used to race stock cars on a 1/4 mile oval and one of the best improvements he made was a wheel and tire combo that was 2lbs lighter per corner. He could not believe what a difference a small reduction of unsprung, rotating weight could make.
One of the odd surprises was after installing a Curtis trailer hitch, a small but noticeable amount of the butt wobble went out of the car in corners. I suspect this is acting as a bit of a backdoor frame brace.
Agreed on the wheels, you really can feel the difference on these cars since you're not working with a ton of power. I had base model Dodge Stealth wheels on my car for a minute when I first got it, but they made the car feel like it was driving in wet cement (heavy wheel and tire combo plus tall, mushy sidewalls). Switching to a relatively light set of steelies was felt immediately.