To start off, dyno tuning is absolutely 100% needed when tuning a brand new new platform.
You cannot find optimal timing, cam timing, fueling, or boost on the street and how it relates to horsepower without seeing a number change. That being said, once you've done a few on the dyno, you get a very good sense of where these numbers need to be.
On the dyno, you may find at at xxxx rpm and x.xx load, the engine responds to a specific timing number that makes the safest and most horsepower at that given spot...and everywhere else on the timing table. You will find the same thing with fueling and cam timing (as well as many other factors, but I am keeping it simple) You CANNOT find these numbers on the street as the engine will start to make less or actually lose power as you exceed that optimal number BEFORE the engine starts to knock. So, just because the engine isn't knocking, doesn't mean its making the most power at that give timing number.
Onto the interesting part. These cars(and many other modern computerized engines) have long and short term learning. When it comes to timing specifically, the engine computer is able to change that based on conditions. The tuner sets a BASE timing table, a maximum timing table, and a allowable retard table which determines how much the ECU is able to reduce timing in the event of real knock(1 or 2* randomly can be normal). He sets this range on the dyno. From that point forward, the ECU will adjust from the RETARD limit of BASE timing able to the MAXIMUM allowable timing all based on ****CONSTANTLY CHANGING CONDITIONS.****
Gas quality, weather conditions (DA), and engine load are all determine what the car runs on timing/fuel/ect. You may find one day the car is down on timing and the next day its up. It changes as much as the waves in the ocean. Constantly and instantly. If you put bad gas in your car one day from a different gas station, the car is able to adjust the timing to prevent possible engine damage. What your car made for timing on the dyno will be different as soon as its on the road and every single run there after. You will never see two logs with identical numbers even when operated under the exact same conditions in the exact same spot.
The reason I explained all of that first is to make my preference of street tuning more understandable. Once your tuner has established a good basemap (which takes a lot of time but good tuners make really good basemaps) then its easy to see that all of the same car setups generally take the same timing spread.
What makes street tuning my preference is not only do you get a much more accurately dialed in map(things like wind resistance, actual ram air, tire friction and other scientific "things") but it does this while putting much less stress on the engine and drivetrain parts. I don't want to give away exactly how I tune, but I ask for datalogs in a very specific manner to not only give me the best data possible, but while also putting the least amount of stress on the engine/drivetrain parts. In doing so, the customer ends up with a much better street car as well as the tuner being able to MINIMIZE failures.
I say minimize because at some point, there may be an engine or turbo hanging on by a thread and as soon as you add more power, even WELL WITHIN safe limits, it may be enough to push them over the edge to failure. That is to no fault of your tuner, builder, or even yourself, its just how things go when modifying past stock. Sometimes, as many of you have found, turbos and engines fail even bone stock. It's just part of the game you play when you modify anything.
Bottom line is, do not fear street tunes. There is no extreme benefit of getting dyno tuned over street tuned other than being able to see your starting point and ending point in the form of a number. Your tuner will be able to give you the exact same level of performance with a street tune and sometimes even more so with a remote street tune. It is much easier to review numbers and make the same changes you'd make in person without the many outside distractions that come from an in person street tune.
Again, all this is based on tuning a range of very similar setups. These cars all run the same exact engines/camshafts/turbos so the tune from one wont be far off from another considering all outside influences being the same.