I'm not sure what this has to do with anything I said. Kunos won't ever make the code open source, and even if they did you would have only spent $4 on the game in the first place so you couldn't feel ripped off if they made it free.
None of them are perfect for me at the moment, I drive a stock JDM Evo X so it has 300 HP while most of the mods are focused on the british FQ360 (360 HP) or FQ400 (400 HP) variants.
The most feature complete one is probably the one made in this thread, but the link is dead. Perhaps there was an update after this that I can't find now.
If you want a 'modded' one with amazing sound, you have to go with
this
My original favourite was this one
here, using
this physics modification. I can't really speak of how accurate this is as it's the FQ400 model that has quite a few differences (god that single exit exhaust in the middle is damn ugly).
One thing that none of the mods can really nail is the S-AWC that the Evo X has in real life. I can't tell you how many times the torque vectoring has saved my ass when I got over-confident and kicked the back end out in the twisties, and I'm pretty sure AC just can't simulate that correctly - especially without manufacturer info or using the stability management assist.