Sorry I missed this - I have been very busy the last few days at work and organizing our club autocross event. If you build big power you will find that the autocross layout creators will even make a layout that defeats horsepower in a big way. I have never dealt with such a tight track. 6 laps per set of Kumhos. Oh my!
Great advice above regarding calling Doug. Prices change over time and that is the best source. Doug is a stand up Viper member and is not out to take you.
That said there was a thread some time ago (maybe even last year) where the prices were stated by Doug himself. I believe he said each individually, the pump upgrade with larger intercooler, and the tensioner.
As to benefit it depends. If you drive in situations where you will heat soak the intercooler coolant supply you will start to lose power as inlet air temps increase and the ECM pulls back spark and adds fuel. A natural response in order to save the engine from detonation (as higher combustion temps lead to increased detonation potential). If you only do bursts of speed, say for a run up the gears the added capacity may not help much. But if you do it on the track, with WOTs over and over, you will definitely need something. Or in high ambients where intercooler efficiency decreases dramatically both the added flow rates and flow capacity will help.
I have gone with th estandard system for 4+ years. At the track I have lived with the power loss as laps build. I am in the midst of installing a second intercooler to be mounted in the back where differential air exhausts. That has several advantages: It adds additional intercooler area, it does not add to underhood cooling issues and it adds significant coolant volume. An added pump in line will assist th eprimary pump as well.I believe Doug has done this on some cars as well.
As to the tensioner that is more mixed. Several people have complained about slipping belts. That would indicate a tensioner issue. I am not sure if it is the parts or the assembly, but Doug's tensioner has received good reviews. Many, myself included, have not seen a need for upgrade in that area.