Spending over $500 to upgrade a piece of your rear derailleur is a bit hard to swallow for most cyclists.  But oversized pulley wheel systems are not marketed to the masses; they are for those looking for a few watts, or seconds, or that small advantage that just might put them on the podium in a tight race.

What does an OSPW upgrade get you?  Let's take what is purported on a case by case basis.   

• Lighter- Made from more exotic materials such as carbon fiber and titanium, 3D printed they are a bit lighter than OEM cages and pulleys but usually by just a few grams.  The performance is negligible here.
• Better bearings- Ceramic in most cases the bearings are considerably better, ceramic bearings have to be nearly perfect to be better though and not all are equal.  But in general they wear longer and produce less friction and increase drivetrain performance. 
• Less chain friction- The basis of the oversized pulleys is that they do not bend the chain as much.  A moving component that does not move as much causes less friction.
• Less wear- By using harder components that are stiffer OSPW systems should last longer and increase chain life if properly maintained.  Because of the larger diameter the pulley wheels do not spin as fast which extrapolates to less wear.
• Better shift quality- Most report better shift quality but this may also be due to less drivetrain suck.
• Adjustable spring tension- means you don't need to have as much tension on the chain further reducing friction.  

If you are upgrading a lower to mid range drivetrain to an OSPW system you will likely notice more difference.  The jockey wheels are plastic and the bearings nothing to write home about.  The plastic wheels can wear and cause more suck.  BUT (and here is the big but) if you are not keeping your drivetrain very clean and well lubricated don't bother.  Just keeping your OEM pulley wheels free of the gunky mix of excess lube and dirt will likely give you the same gain in wattage as an upgraded OSPW system.  In short, if you are not that person that demands a clean and well maintained drivetrain; don't bother.  An OSPW system is for those that are looking for a slight increase, edge, and improvement in their drivetrain after they have already maximized it by maintaining their chain, bottom bracket, cassette, chain rings, etc..