I'm going to write a novel on this one...
This platform is 4 years old, and no one has a legitimate idea what the stock engine is capable of. People have decided that 400-450wtq is a safe and reasonable limit, and that has become the general consensus, but there is zero data to back that up. It's commonly accepted, but not fact. I've been above 400WTQ for awhile, and my oil analysis results show wear levels below what many people see on stock cars. Keep in mind these engines are closed deck iron block and fully forged from the factory. That was a major consideration in why i bought an MK7...it's a recipe for reliability at high power levels.
Determining the limit of an engine is very complex, and i've been around this stuff long enough to know that the commonly accepted limits are typically bullshit. Many years ago i was into 4G63s, and in the beginning everyone thought 450-500whp was the safe limit for a stock motor. People who wanted to go above that built motors...which ended up being a complete waste of time and money. Eventually, people started making 600-700+whp and buzzing 8,000+ rpm on the stock bottom end (pushing street cars into the 9s @140+). The first 9 second stock bottom DSM had 100K miles, 900 passes at 550awhp, and another 50 or so passes above that. Eventually the block cracked, which wouldn't have been prevented by building the motor. His secret was an aftermarket ECU that provided much better knock control than the factory one (back then stock ECUs were junk, now they're incredible).
Same thing can be seen with the GM LS engines. Stock bottom end 4.8s have made 1200hp, 5.3s have been over 1300hp, and 6.0s have been over 1500hp...and that's with Gen III engines. The Gen IV was a significantly stronger design, the 5.3 has been 7.16@190mph, yet i have friends that have blown them to bits with slow ass 650hp cars. They never reached anywhere near the structural capability of any of the components, but they failed to control knock (cheap intercoolers, pump gas, etc).
Controlling knock is the single biggest factor in keeping engines alive. Turbos are the most gentle power adder, so even average internals are capable of some pretty serious power if setup and tuned correctly. Keep the IATs down and the octane up, and you'd be surprised what engines will take.
I have a turbo GSXR with a factory sealed motor. I was told over and over it would blow up above 6 psi. I'm running 11psi with 12.5:1 compression, and not a single degree of timing removed from the original timing map. The secret is controlling knock and combustion temps by running E85, and high viscosity oils (50wt).
Don't get me wrong, built motors are awesome. I had a built 4G63, and the thing was tough as nails, but at the end of the day if i had put the money in other places, i could have gone faster on a stock bottom end than i ever did on the built motor.
I don't like the idea of building a motor because you think you have to, or because the general consensus is that you should. There are a million factors that determine how much power you can make safely (intended use, power adder type, temps, knock, ECU capabilities, and the tune, to name a few). Consider those carefully and go from there. You can't look at where one engine failed and think that's the limit for the components, engines fail stock.
I will say, a built motor is in a way, an insurance policy. If you're pushing stock components to high power levels, a small problem can cause a catastrophic failure, where as a built motor will tolerate a lean condition or knock event MUCH better than a stock one.
Also, be very careful who builds your engine if you go that route. I've seen brand new engines from big name builders (top dollar stuff) fail terribly because of builder error. Hell, one of them had a paper towel in the oil pan which clogged the pickup, lost the engine and both turbos. There are several machine shops near me, but the only one i'll go to is a 2 hour drive. Also, good machine work shouldn't be terribly expensive. Some machinists are like some tuners, they try to convince you there is magic involved, and way overcharge you for their particular brand of wizardry. Simply bullshit. The choice of your machinist is far more important than the brand of rods or pistons you choose, so be smart.
