The 19" tyres have lower profile and this is an advantage on fast corners due to the more rigid sidewalls. On road it is a complete waste of money because the tarmac is not as smooth as in the track and you will see no advantage. I ordered my PP with the (standard here) 17" Brooklyns. And I must say that the PP is phenomenal in wet hairpins. I found it out the other day as it kept its line and pushed me to the inside of the turn while I was feeding the power before the apex. Money well spent, very happy indeed.
You're right in theory, if and only if, the suspension maintains a consistent contact patch under load i.e. when the suspension is loaded there are no changes in geometry... which is unlikely. Then there is the discussion about mass, how does the mass of the 18" wheel/tyre combo compare with the 19"?
Side wall stiffness is one (small) aspect... bigger is not always better when it comes to grip, never mind handling...