What does 500+ mean? Like around 500 whp or 600 whp?

All those turbos should make the power. What kind of Holsets are you talking about and what is the A/R on all those turbos? It might be helpful to find some compressor maps and see if your turbo is capable of flowing what you need it to without being too laggy.

Any size valve is fine. I would recommend Supertech valvetrain parts. There are a lot of good engine managements. Who is tuning the car? Find out what they are comfortable tuning. I run Neptune on my setup but AEM EMS, Hondata, Crome, and eCtune are all decent tuning solutions. Windage tray is a good idea. You can go with a lightweight flywheel but it will probably make your traction problems worse. K&N cone filter...it has been proven to make the power over screens or no filter. For 500 whp, I wouldn't go any smaller than 1000cc injectors. Rings that come with the pistons are what you want to run.