Overall increase in powerband
Slightly better mpg (though I havent seen it, any increase was negated by my lack of willpower to drive it harder with its newfound power)
Manufacturers typically build alot of headroom into turbocharged platforms to make them robust and durable for long term use. This means as tuners, theres lots of headroom to make things faster without having to build a new motor to withstand it. Boltons/Tune will not have any detrimental effects on your car, pending the tune is not some half-assed thing from a no-name tuner that just started out last month.
The only down side is that you may encounter premature engine failure depending on how it was tuned and how you drive it, but overall tunes are fairly safe when you use a reputable tuner.
Chips/piggy bakcs, on the other hand, can be dangerous since they tend to be static and overly aggressive (and built by non reputable tuners). If you're talking about the EEPROM chips, then you must be sure you get the tuner the EXACT specs you're using and never change anything after that. But, we no longer use such antiquated methods of tuning, so you're fine.