I personally don't think the S4 is that underrated at all really. I mean it weighs close to 4000lbs, has "333hp" but has a slick dual clutch and AWD. The 4.4 0-60 is purely due to LOTS of grip and decent torque. Its only trapping 105mph in the 1/4mile, which seems right considering the power/weight and the fact it has a dual clutch. Think about it, stock STi weights approx. 500lbs less, but has 35 less horsepower, traps at 100mph. 33hp and a dual clutch nets you another 5mph. If it truly had 400 like Lieberman likes to suggest, the thing would be trapping close to 110 stock... Just saying I don't think its as underrated as people think, it might be making 350-360hp MAYBE, but nowhere near 400.
When it comes to the 45, I would like to see it a on a good old Mustang dyno, they seem to be the only ones that give a really accurate representation of what a car is making at the wheels. A45 video on youtube shows it going 0-200km/h in approx. 18 seconds, Car and Driver listed the 0-130mph time in just 19.2 seconds, which is actually faster than a Mustang 5.0!!! Definitely proves the engine is underated, considering the CLA45 that C&D tested weighed actually 20lbs MORE than the mustang.. And supposedly makes 54 less HP!
As to the engines reliability, the pistons and crankshaft are forged, as someone else said the rods aren't mentioned and could be a weak point. Considering the given information, upgraded rods and rod/main bearings, and this engine should be good for over 500 at the wheels. I've been in the Subaru tuning scene for years, and with a 'built motor' consisting of just pistons, rods, bearings and head studs, those motors are good for over 500 easy with a large turbo and supporting mods. You guys are right, the engine probably can't take a whole lot more of boost without needing to be sleeved. The thing is, if people wanna make serious power, they are gonna need to upgrade to a larger turbo, run LESS boost than stock even to make a lot more power, but then turbo lag will be added into the equation, as it's impossible to retain the stock powerband. People are gonna have to decide whether they want a more usable daily powerband, or a lot more power, but a lot more lag with it!
EDIT:
Here is the stated video, car seems to pull much harder in gear during the second attempt, assuming the car wasn't fully up to temp in the first attempt. Enjoy