On 13 April 2019, an experienced Cessna 525 pilot almost lost control shortly after takeoff from Bournemouth when a recently installed performance enhancement system malfunctioned. After a six minute flight involving a potentially hazardous upset and recovery of compromised control, the turn back was successful. The Investigation found that although the pilot was unaware of the supplementary procedures supporting the modification, these did not adequately address possible failure cases. Also, certification flight tests prior to modification approval did not identify the severity of some possible failure outcomes and corresponding Safety Recommendations were made to the system manufacturer and safety regulators.