2005 c230 tire question

I have a 2005 C230 BENZ Sports 4-door. My stock tires are 18" Z-rated 92Y-Load Factor. The doortag states that I should get a 91-Load Factor. Costco tires informed me that the BF tires that they have are a 88-Load factor and told me that this is fine if I add an extra 2PSI to all the tires. Is this true? Does this effect the performance of the tire/tread life/anything at all? I don't really understand too much about tires, so does a 3-load factor difference matter? Thank you