Carver Custom's updated magwell for the M&P strikes a happy medium, making it our No. 1 recommendation. It's big enough to be a full-on competition magwell, yet relatively compact compared to others of its type.
High value for the price
Legal for most practical competitions (except carry optics+production USPSA, SSP division IDPA)
Designed to work with Taylor Freelance basepads
Works with virtually all basepads -- including stock magazines.
Same profile as Carver Custom "Open" magwell, but relieved inside to accept factory basepads.