Freaking EA deleting my post because its spam LMFAO I gotta write this a second time but im not gonna drop all the info this time.
From my experience, High Heat grants ENGINE Parts that are 1 level higher than your current unlocked category ranging from Pro on up.
Pro to Super ( Or Rep Unlock )
Super to Super Plus ( Or Rep Unlock )
Super Plus to Elite ( Or Rep Unlock )
Elite to Elite Plus ( Or Rep Unlock )
Elite Plus to Ultimate ( High Heat Only )
Ultimate to Ultimate Plus ( High Heat Only )
Game Difficulty affects overall part unlock as well and what place you finish the race in, 4th being the lowest I believe.
High Heat Rewards breakdown
Level 3
Nos Kit High chance
Forced Induction Kit High Chance
Better Engine Part Low Chance
Chance to gain engine part is higher if done on Hard Mode
Level 5
Nos Kit High Chance
Forced Induction Kit High Chance
Better Engine Part Better Chance
Chance to gain Engine Part is greater if done on hard mode
Going from Ultimate to Ultimate Plus is the same chance I feel except for Level 5 Heat on Hard Mode.
Heat 5 Races on Hard gives me more Engine Parts that boost performance rather than NOS or Forced Induction kits.
https://imgur.com/a/K4Xtb8i