Are there any compromises going down to a 15" wheel?
Overall size of the tire is essentially the same - circumfrance is 84.4, compared to 84.8 for the 16". The walls of the 15" tires are larger that the 16" so there will be some compromise of performance- I think it will be minimal.
How much did Costco charge to do the installation? Was that $60 rebate for each tire or total for 4 tires?
60$ rebate for four tires, applied after tax. I haven't had the tires installed yet - I bought the tires and had them mounted on the rims. I will be taking the XTrail in after the tire sale is over to have them installed. I will report back here after they are installed if you want.
What steel rims did you end up going with?
The rims are from Costco, I guess they are designed for Nissans, not the universal fit rims. Other than that I don't know anything about them.