You don't really want to go to a thicker sway bar up front. The whole point of going with a thicker rear bar is to bring the balance of the car away from understeer and increasing the roll stiffness in front encourages more understeer.

Oh btw, the '98-up JDM ITR did come with a 25 mm front bar. The '99-'00 Si coupe does have a rear sway bar as well, not sure if it's the same 19 mm as the Integra or something thinner like the Civic EX.

Regardless, the stock GSR front sway bar is the way to go.