Re-do your math, you're mixing positives and negatives, I think?
While the 15x8" is 2.5" wider of a wheel, the whole "package" is only 20mm wider, or about 3/4". The 185's "balloon" off of the stock wheels while the 205's and the 8" wheels are the same width.
Look at it like this, the tread width and section width of the tire are the only things to worry about as that's what gets close to the fender lip. The stock 185 tire has a 185mm tread width (give or take, each tire brand/model will be different) which is 7.3". The 205 tire has a tread width of 205mm which is 8". So even with making the backspacing/offset equal, the increase in overall width is only .7" or about 18mm. That's less then 2cm and less then 1". There's a good 1.5" in between where the tire would be and my fender lip. The only real difference is where the wheel itself is in relation to the body, and with most +15 15x8 wheels you will end up with a deep lip so it's only the actual "rim" that will be out the additional amount, and there's plenty of room to grow in that area from the "+" wheel!
I attached some pictures
