Not counting legal emissions reasons, are PCV and EGR necessary to run modern engines? Are there any performance or efficiency gains with PCV or EGR systems?
- Vacuum helps scavenge evaporated contaminants in crankcase.
- EGR improves fuel economy and lowers engine temperature. (this has been covered a few times in excruciating detail already).
- Engine control modules are programmed to work with these systems, so modifying them would be counterproductive.
As @mmj said, emissions aside the PCV valve provides positive extraction of blowby products in the crankcase. Before PCV was adoped we had road draft tubes which didn't really provide effective venting. That's one of the reasons that engines would wear out before they even reached 100K miles.
EGR debuted in 1973 to reduced NOx emissions by lowering combustion chamber temperatures. Before computer controls and even catalytic converters were introduced this, along with other modifications needed to meet the then-new standards, resulted in a noticeable drop in power and gas mileage. People would commonly disable the EGR valve and other crude emission controls back then. Today EGR is an integral part of the engine control system and should not be tampered with. (Not all cars have an EGR valve. Valve overlap is used instead.)
Regarding the use of EGR and PCV valves instituted in the 70's as emission control devices, I seriously doubt that you'll find these do dads on any race cars, IMO.
Of course not because there aren’t millions of them driving on the road every day.