18" are the best choice for handling and ride whether you are on the road or track.
On track you can take on the kerbs without unsettling the balance of the car.
No race car runs such low profiles, after all the tyre is the suspension between the wheel and the tarmac thus 18"s due to the deeper side wall gives great flexibility and a better contact patch which in turn = grip.
18" BBS LM'S are lighter than the 19" OEM wheels its just the tyres that are heavier. I haven't come across any one who has got skill requirement to make the different out of the weight issue over the grip level what 18" offer.
T$