My understanding is that any BTO is obviously covered for replacements under defects, etc., however, if you are not happy with it and simply want to return it for a refund, they will most likely charge you a restocking fee since you didn't buy a stock model.
Please note that Apple does not permit the return of or offer refunds for the following products:
Personalized products, such as engraved iPods and iPads
Opened memory
Opened software*
Electronic Software Downloads
Software Up-to-Date Program Products (i.e., software upgrade subscription product)
Apple Gift Cards
Apple Gift Packaging (origami style gift box with ribbon and greeting card)
Apple Developer Connection Products (Membership and Technical Support)
I just read the Apple policy. It just says:
What I can't understand is if a BTO iMac has to be considered a Personalized product
I believe that they are called CTO (Configured-to-order) Macs, and would therefore fall under Apples Custom-Configured Product policy found here:
Custom-Configured Product
Apple offers certain products that may be custom-configured to your specifications. Please review your order carefully. Once your custom-configured item has been prepared for shipment, it may not be changed or canceled. Learn more about Apple's Return and Refund Policy.