You run out of injectors at around 540-550ish, then out of turbos at 570-580ish.
From there on it quickly gets increasingly expensive.
Can't offer any opinion about off-the-shelf tunes as I didn't really dissect them (apart from MTM one, but that's pretty much irrelevant as it's NLA).
And yea, generally the main difference is that off-the-shelf tune is usually made for the wide spectrum of cars out there, while custom one will be tailored for your specific car.
Obviously, depending on the competence of the tuner you choose, you can get some really bad things inside (holds true for both off-the-shelf and custom).
For example, couple of very common "bad practices" are:
- not really turning off diagnosis of missing hardware (emissions related, usually), but just suppressing the display of DTCs.
- raising or even completely removing safety related mechanisms in the tune (overboost protection, knock control, etc) - this is also known as "italian tuning".
This can even sometimes make your car slightly faster than the "elegantly" tuned one, but the price for that is too high: longevity of components can be dramatically reduced.
I personally prefer to leave all the safety mechanisms in place and just alter some of them if absolutely necessary.