Focus on some headwork, and fuel. Those are the 2 biggest issues with this car. There's a company called DCR that's looking to absolutely destroy the 409hp record and is actually strapping on a PI fuel system so they get all the time in the world to get the fuel where it needs to go. Not sure what they're doing yet with the MAF and throttle body, but expect some serious numbers if anyone's going for them.
I'd say if you want to do this right, prepare to invest around $10-12k. I'm not going to say you can't do >409hp, but prepare to invest a ton of time and money. You're not going to reach that mark with bolt-ons.