Have gone through the wheel threads and various others trying to find what is the limit for rear sizing. Looking at a pair of 16x8 +20 and measuring from my current wheels which are 16x7 +40. They look/measure as though they will just stick out which im not a fan of. But I have found a few pics of mr2s with similar (17x8.5 +30) and they look to fit....
Anyone running something similar to comment?

20 mm offset may not be ideal.
 
I test fitted the popular Nissan GTR 16 x 8" wheels that are 30 mm offset on the rear of my SW20 that is lowered about 30 mm from stock.
1. I found with 225/45R16 tyres there appeared to be no rubbing inside the fenders. (but very close)
2. With 225/50 R16 tyres they rubbed noticeably, and the fenders would definitely need rolling with them.
 
On the rear I now run 16x8 Simmons wheels with a 35 mm offset.
1. With 225/45R16 tyres no trace of rubbing.
2. With 225/50R16 tyres the tyres touched the fenders on large bumps, so I needed to slightly roll the fenders.
 
IMO, the perfect offset for a 16X8 rim would be 35 mm. 
 
 
 

Thankyou for the in depth reply!! Exactly what I was trying to find out. Sounds like they aren't gonna work as I have 225/50/r16.