So I've conducted this little test.
Test1:
Circuit: Silverstone
Laps: 50% Race
Mode: Single Player
Car: McLaren
Laptimes in RACE: Constant low 1.34s and high 1.35s
Test2:
Mode: Online
Circuit: Silverstone
Laps: 50% Race
Car: Force India (Performance set to equal)
Laptimes in RACE: Mid 1.36s...