You'll want to be careful porting your stock exhaust manis. They're basically like a tublar/log manifold inside of that outer skin, so the walls aren't terribly thick (it's not a big one piece casting and that whole section from the inside to the outside can't be bored out however you want.) I personally wouldn't deal with porting the exhaust mani, and lots of people have made big power with the stock manis (a Tial car did 600+whp.)

Yeah, like was already said, dyno numbers are just numbers that should really only be compared to the same car on the same dyno, in relatively the same conditions. Also like was said, those claimed numbers are either chp or bhp, not whp like everyone else is talking about. With all of the smaller stuff like an intake, headers and high flow cat it's hard to put specific numbers on that stuff (especially when our stock intake works so well.) Mostly people have numbers for the different stages (1, 2 and 2+ which is a chip, catback, and downpipes respectively.) Don't worry too much about the specifics of the numbers, but know that those 3 mods will net you the most gains for your money.

I've heard of a couple of people that have had issues with the APR bipipe. Some say that they had pinhole sized leaks from the castings and some others had issues where the bipipe meets up with the TB I believe. Tons of others have had no issues. As far as the GENX or now ARD bipipes, they're the biggest you can fit in there (2.25" I believe) and they're just a uniform tube. The stock, RS4 and apr bipipe all change cross-sectional area and shape multiple times throughout their length.

As far as breaking rods, I'd go with a rather conservative tune if I were you and were worried about the engine. It doesn't really seem like you're worried about if/when it goes, so no stress there I guess. People have broken/bent rods with around 500whp (and even less) before, also there's people who are successfully running a little more, so you just need to calculate what you're willing to risk. Axles can break, but I don't don't think you'll need to worry about that at RS6 power levels, and I bet the rods go before the axles.

I personally wouldn't build the heads with RS6's because they'll be perfectly fine with a 7200-7500RPM rev limiter. I built mine because I had to rebuild them anyways because the motor I bought had some bent valves. You can throw rods in and leave everything else stock and go for broke with maxing out the RS6's, it doesn't require built heads etc. And you definitely don't need to replace pistons, unless you want to go bigger displacement (or lower compression ratio and don't want to do a headgasket spacer.)

 

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