This is a partial list of rare and super rare mods in Forza Motorsport 6, as revealed by opening 11 of the most expensive in-game mod packs, each costing 300,000 in-game credits to purchase. Not all of the mods are listed here, most likely, but it should give you a sense of the spread and variety of the rarest mods available in the game.
As a quick refresher. you can apply three mods during a race. All of the mods below not marked Crew or Dare, which are special mods where you can apply one of each type during a race, are single-use Boosts that are disposed after the race is over and done. You will lose these Boost mods if you quit the race. Crew mods increase your vehicle's braking and power and decrease its weight, while Dare mods give you extra credits for completing its objective successfully.
For more information, you can read Jonathan Leack's preview as well my preview which explores the first two hours of the game.
Braking Engineer | (Crew) +20% Braking / -5% Weight.
Braking Expert | (Crew) +16% Braking / +3% Power.
Chassis Expert | (Crew) -7% Weight / +3% Power.
Driver Bonus | +60% Driver Level XP.
Driver Expertise | +60% Driver Level.
Experience Bonus | +4,000 XP.
Ghost | Your car has no collisions for the first lap of this race.
Manual With Clutch | (Dare) Manual Transmission With Clutch. +20% CR.
Manufacturer Expertise | +60% Affinity Level XP.
Power Train Expert | +7% Power / -3% Weight.
Pristine Cornering | Earn 3,000 CR for Perfect Turning up to 18,000 CR.
Pristine Drafting | Earn 3,000 CR for Perfect Drafts up to 18,000 CR.
Pristine Passing | Earn 3,000 CR for Perfect Passes up to 18,000 CR.
Severe Grip Reduction | (Dare) Grip decreased by 9%. +20% CR.
Severe Setback | (Dare) Start in 22nd Grid Position. +20% CR.
Severe Shift Time Increase | (Dare) Shift time increased by 0.75 seconds. +20% CR.
Severe Weight Increase | Weight Increased by 8%. +20% CR.
Sim Damage | Simulation Damage. +20% CR.
Superior Buyout | +60% Credits.
Superior Grid Position | +3 Grid Position.
Superior Preparation | +60% XP Boost.
Adept | +7,000 Driver Level XP.
Chassis Engineer | (Crew) -10% / +12% Braking.
Dark Horse | (Dare) Power decreased by -12%. +30% CR.
Elite Grid Position | +4 Grid Position.
Grip Engineer | (Crew) +12% Grip / -5% Weight.
Manufacturer Endorsement | +7,000 XP Affinity Level XP.
Overtaker | Earn 4K CR for Perfect Passes up to 32,000 CR.
Powerstrain Engineer Crew | +10% Power / +12% Braking
Slipstream | Earn 4,000 CR for Perfect Drafts up to 32,000 CR.