polar993 the voltage figures you quoted with engine running are spot on so no issue there. 13.7 volts before starting is high but probably due to being on charge just before - nothing to worry about. For reference typical lead/acid (OE) battery off load would be 12.7 volts fully charged.
Further thoughts. What is the capacity of this battery (measured in amp/hrs)? The fact that it's claimed to deliver 880 cranking amps has no relevance if it's capacity is so small that the immobiliser can drain it in a week!
For reference the factory Banner battery you replaced is a 30 amp/hr one. Although inadequate capacity, my M3W would start with this after 3 weeks despite immobiliser drain. The suggested current drain measurements won't tell you much other than there is some due to immobiliser. It may burn out a perfectly good multi-meter though if you don't know what you are doing, not to mention possibility of shorting battery!
As already suggested the manufacturers should be the best source of answers.
