So I just noticed that the passenger side rear of my R53 has significantly less wheel gap (0.5") than my driver's side rear wheel gap. Any clue what could cause this?

Suspension set up include H-Sport Springs (only a few thousand miles on them), and H-Sport rear control arms. I had an alignment done a few months ago and everything was equal on both passenger and driver side.

 

There's only a few possibilities:
The springs aren't fully matched in terms of ride height (one shorter than the other side by a little).
The body, fender, suspsension parts have some assymetry (unlikely if they were
the same height a few months ago and no accidents bumps or big jolts have happened).
Something put in wrong turned around or missing regarding the strut mount assembly on one side only.
You're not parked on a fully level surface for the current measurement.
You've got different size tires on either side (!hope not!).

 

Could be alittle of everything above along with the black fender trim itself being off some side to side..
If one side is pushed up all the way and one side is pulled down all the way, don't think it would be 1/2'' but could be some of it..
There is some play in the fastners...

 

Miami, I was freaked out by this too...thought I had the same problem until:
1) I realized the street I was parked on was crowned
2) My trim was a little off after I got backed into by a douc*** in a F250.

On a level surface I'm less out of wack, and once I get everything realigned, I think I'll be OK

 

Had the same thing happen just recently. Found out in my case one of the front shocks was not seated all the way in the steering knuckle "cup".

 

Short Wait, I'm assuming you noticed some "pull" while driving, along with the difference in wheel gap?

 

You might have a strut that has failed or some strut tower issues....look at the top of the strut tower...fronts are known to deform....rears are kinda rare...

 

im taking it into the shop... the car doesnt pull at all, perfectly aligned a few months ago. springs were new h-sports. im thinking either blown strut or something with the strut mount... OR...

When I installed the lower adjustable rear control arms, one of the stock lower arms was bent so it could be that the upper rear control arm is bent as well, but i think that would only affect camber, not wheel gap (and consequently ride height).

We shall see!

 

Sounds like if you have had an impact big enough to bend suspension parts...other dammage is likely....that would explain the wheel gap.
Remember, suspension parts work in 3d...a change in one plane DOES affect the other 3...some items are mostly one plane, but a good suspension person can explain how it also changes the geometry on the other 2...all changes have a effect. If everything is working fine, and tires are ok....it may get very $$ to fix your cosmetic issue...