I've taken corners fast in my IS200T F-Sport pretty fast, especially on the spiral offramps, and never lost traction let alone understeered. I have some offbrand tires that came with the car CPO, called Barums. For you to be continuously understeering you're definitely going too fast

 

If you're pushing the limits of the car in autocross or on track, understeering is probably the norm. I think that's what OP wants to fight against - at-the-limit understeer. Of course, understeer on public roads just means you're doing too much, to your point.

 

This is what I'm thinking about. But it seems a bit abnormal to me that your rears are wearing on the inside, maybe its an alignment issue? It could be that the tires are too stretched, but I'd imagine that would cause accelerated wear all over the tire. What exactly is the function of spacers?
Also, how's the grip? Does the car feel more neutral?

 

Correct, lol. I'm actually glad autocross exists, if anything it promotes safer driving on the road, since you can get all that adrenaline out on the track without punching a serious hole in your pocket like track days.
But yeah, even though I understand that the IS is front heavy, especially with that chain drive transfer case thing up front for AWD, it pushes the limits of the front tires too soon while on track. BMW's xdrive is technically a similar rear biased system, but my buddy's 428i xdrive doesn't have the same tendency, even without the M sport suspension thing. Its also not a lot lighter, probably 100lbs. It can be made to rotate at corner exit with easily controllable oversteer. (Its a lot slower overall but that's not the point lol). It is however, running a 225 all round square setup. Which brings me back to tires....

 

My rears were wearing on the inside from the get go, haven’t checked michelins yet but 100% sure they will. Alignment is fine its the camber which is not adjustable. Unless i buy camber kit. I read this forum and some people experience it too with awd cars and my dealer said it is what it is aggressive set up from the factory bs.
Spacers is just to push ur wheels out for more aggressive look. Not needed at all but I might use it for the rotation of the wheels ( back to front) because rears won’t clear the callipers without spacers.

Tires are ok, great grip on the wet road. I keep them over inflated a bit at 39-40psi cold as paranoid about stupid inner wear so there is a slight understeer in hard corners but I feel if psi would be set lower at oem specs grip would improve.

 

Guess maybe the F-sport wheels are different?
I had swapped the front and rear on my 2014 IS 350 AWD while trying to figure out front wheel noise (this was before I knew about the soft LCA bushings).
I didn't drive it long this way, but I didn't have any issues with the wider wheels up front.