This is what my rear looks like:

 

That was the initial recommendation on offsets from BC, but he adjusted that to +32 on the rear when I noted I wanted to run 305s.

 

Thanks for the pics!

This has opening up a can of worms for me! So the +42 front and +28 rear that BC recommend is really a little too close for comfort pushed out? Both your pics above look to me very close to the arch.
Your thoughts and opinions are definitely really useful, gonna drop a lot of money on these BC’s!!

 

The +32 I've got has given no rubbing whatsoever and I live on a street with speed bumps. It's those and the driveways I need to traverse that have kept my lowering springs in a box instead of on the car.

 

I would personally stick w/ the 32's if you intend to run 305's. since I run the 295's and 11" rims, the shoulders on the tires are a bit more rounded than stock, giving me a little more room to work with...and I'm on VAP springs. but full disclosure, I have not taken a drive yet since installing the spacers to fully confirm since I am not able to take the car out of the garage due to construction but I'm pretty confident in my math when I chose the 5mm spacers. or you could be more conservative and go w/ a smaller offset, giving you the ability to add springs and still be safe, and then adjust it via spacers later. food for thought.

 

I agree here - 32's if going with 305 tires. My BC forged wheel offsets are 36 front (255), and 21 rear on a 10.5 (295). No rubbing and it looked great (my previous R - I now have an SVR). But, if upsizing to 305 width, you should be more cautious with the offset - 21 would probably rub, 32 might be just right.

 

I've hit big enough bumps with the +32 to claim that it's safe. The +28? I can't say

I don't think lowering would change that, and I really don't see how it should. The spring does not control the wheel position through travel, but does determine where in that travel it will be. With a coil over shock suspension, removing the shock assembly allows the clearance to be checked through the entire range of travel and for the front, steering angle as well. That doesn't account for bushing deflection but it's a safer way to check that a drive test.