Inner tire wear is only an issue on the front. Replacing the lower control arm bushings with the ones from the RCF/GSF will reduce that.
If the issue is the rear, then likely it's an alignment that's out. When you have new tires installed get an alignment done.
The rear has Toe adjustment only...setting it to the factory alignment specs for the rear is fine.

14-15 IS250 are the same

This is correct; HOWEVER, if you are having issues with inner tire wear on the rears for whatever reason, buy SPC Camber Arms for the rear. That's what I installed to combat the extra camber from lowering the car.

That is also true...although I don't think he's lowered it yet...but planning to.

If you lower it more that RSR Downs equivalent, then you may want to look into the SPC arms or something similar for the rear.
I guess the first thing to determine is, what kind of wear are you getting on the rear? If everything is stock then I'd say it's likely Toe where, but you need to start with new tires and alignment before thinking about next steps.

Yeah definitely shouldn't be getting camber wear on the rear if on stock. I would def look into the toe on the rears like stated by Sasnuke.