I’ve said it before but as long as Steve Harris is at the helm, it doesn’t really matter who the producer is. I imagine a more hands on producer like Sneap or Roy Z would fight Steve’s wishes until finding themselves out of a job. Then you have an album produced by Steve without any other input.
Shirley is the best guy for the job. He knows the band as musical personalities and is able to make really good sounding albums with minimal processing (see Counterparts by Rush). If Steve would stay out of his way, we’d have more albums like Brave New World, which is one of the best sounding modern metal albums.