When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I have been running 15mm spacers behind my 6UL's for a number of years now since putting flares on. You have to be more careful about centering while getting the lugs tight but no issues other than the car turning faster lap times after putting the spacers in.
The spacers themselves don't add to the stress. Its the change in offset that does (so it would be the same if he ran 40mm offset w/ a 5mm spacer or 35mm offset).