impact of costco on wow and wes

I saw this article comparing prices for warehouse clubs with full line supermarkets in the US:

http://finance.yahoo.com/family-home/article/107083/Warehouse-Club-Face-Off-the-Overall-Savings

The question is how damaging Costco is for WOW / WES. I believe certain price sensitive consumers (+SMEs) will do megashops at Costso every month or quarter. This may reduce sales at WOW / WES by a couple of % each nationally, which is quite significant.

Thoughts?