There is no benefit to going to a 19 inch rim.
It's been tested on other cars a few times by magazines and if i remember correctly, once a car reached 18's, there was almost no difference in handling to a 19.
Lets not forget too, 19 is an oddball size, most good tires are not made in the 19 inch diameter.
Last, but not least, 18's on a fox leave you pretty low on sidewall, 19's would be worse.