The Wilwoods rock, but as with any performance part, proper pre set-up is a must. I have exparienced all issues you guys are seeing, in other divisions. Mainly when residual valves are used which is not the case here. Biggest step Legends drivers are skipping during the Wilwood upgrade is not machining the rotors before install. Also, 80 grit sandpaper will not cut the mustard for the final sanding of the rotors before install. Second is improper shimming of the pads if an issue is seen. Attempting to shim the pad after the issues are noticed will not cure the problem entirely because the Celica rotors are junk. They are not performance rotors. The rotors are made of a very porous metal. If sanded after cutting with heavy sand paper, the porus metal of the rotors will have valleys and hills in them so to speak and cause heat buildup and chiming. That is the number one cause of the pads chiming when they are hot as these bad spots are amplified. Secondly, when you apply the brakes the brake pads contact the rotor resulting in friction (heat). That heat softens the brake pad compound so you have compound transfer to the rotor. The heated compound actually fills in the soft pores (valleys) of the rotor. After a few stops the brake pads are no longer touching iron, they are touching a thin film of compound embedded into the pores of the cheaply made rotor (not meant for this performance application).
A few quick tricks to attempt to cure the problem is beveling the leading edges of the pads, sanding the snot out of the rotors with fine sand paper (I have seen guys heat the rotors and then sand them) before use and proper shimming of the pads before the first use. Also after a good hard run, flip the inner and outer pads in the caliper. Rotors may not look warped but the soft castings of these stock rotors will be amplified under hard heat build up and amplify brake issues. INEX rules state no drilling of rotors but most likely this is a cure for hard braking issues that will be amplified with these performance brake calipers. Drilling, chamfering the drilled holes and balancing the rotors should allow any pad compound available out there for these brakes to work on this light chassis thereafter as it will give the generated heat a place to go and allow a cooling area within the rotors.... On the other hand, it does not state that you cannot drill the pads within their rules so maybe that would be a quick relief of some heat along with the beveled leading edge of the pads
The new brakes are sweet but the stock Toyota metal composition of the rotors is improper for this caliber of a caliper... rotors is the issue and if upgraded would be the cure