i was on zzperformance.com and i saw an egr backoff plate. what does it do and how does it help performance?

Brings in cleaner air, or something like that.

To get what the block-off plate will do, u have to know what the EGR does. The EGR valve is vacuum operated, with the use of a solenoid, and it opens to allow spent exhaust gas back into the intake to be re-burned on the next compression and power stroke. All this is in the eye of emission-control. By doing this, it introduces a lot of hot air into the incoming air. Hot air= less power. By putting a block-off plate in place of the EGR valve, you prevent the hot air from returning to the engine, hence using only cooler, denser, more powerful air in the air/fuel mixture. However, there are downfalls. One, you will have a never ending SES light on the dash. Second, the early non-EGR 3800 engines had a problem with burning exhaust valves. On a SII motor you may not have this problem, but the possibility is present. I hope i explained it ok, anyone else have somethin to add?

You don't need it unless you dig really deep into an L67 (specifically) with an intercooler.