2.5" piping may be too large for your application. consider 2.25". 2.25" can handle about 170whp or so. Considering that a yaris only has a 1.5L engine and about 106hp stock, 2.25" piping should be more than sufficiant and will leave you with some backpressure. 2.5" piping will leave no backpressure and will probably cause you to lose power compared to the 2.25" piping. just my $.02