Wagon has 240k mi (140k mi tuned stage 2, only one cat left). I Seafoam before every other 3k mi OCI. Replaced the (now sole) O2 sensor once since tuned. Failure was not proximal to Seafoaming. Never had an issue. Except for Seafoam testimonials, I feel that most additives are snake oil.
Adequately designed studies are rare. The BG Chemtool claims are true that it dissolved polystyrene - it's got acetone in it! Duh!