Not exactly a comprehensive list but a list none the less.
More Horsepower Than Advertised - Cars That Secretly Make Tons of Power
L88 Corvette
Rated 435, made at least 100 more than marketed.
Nissan Skyline GTR
Rated at 276 do to Japanese regulations for that time but made closer to 320 (R32 at least).
Ferrari F40
478 officially, believed to make well over 500.
Boss 429
375 rated, closer to 475-500.
69 GTO Judge
GM claimed 370 but that was apparently made up. Made closer to 400+
BMW M5 (current)
560 advertised, independent dyno shows 620
Porsche 959
450 rated, likely way past 500
68 Firebird 400
325 claimed, likely closer to 360
More Horsepower Than Advertised - Cars That Secretly Make Tons of Power
L88 Corvette
Rated 435, made at least 100 more than marketed.
Nissan Skyline GTR
Rated at 276 do to Japanese regulations for that time but made closer to 320 (R32 at least).
Ferrari F40
478 officially, believed to make well over 500.
Boss 429
375 rated, closer to 475-500.
69 GTO Judge
GM claimed 370 but that was apparently made up. Made closer to 400+
BMW M5 (current)
560 advertised, independent dyno shows 620
Porsche 959
450 rated, likely way past 500
68 Firebird 400
325 claimed, likely closer to 360