My Dad used to have Continental Premiumcontacts on his E39 which had a slightly bulgy look, not to a massive degree but enough for me to notice it. I can upload a picture if you like.
Make sure the sidewall height is the same, remembering that the number is actually a percentage of the tread width. So a 205/55 is 112.75mm tall, and a 185/55 is 101.75mm. You'd need a 185/60 to get a 112mm sidewall in the narrower width.
