Logging is very cool but usually come to a price. How the performance are affected by logging?
By default logging will come into play only when an error happens. This means that is the packages has no errors logging won't impact on your performance.
Dataflow Profiling is amazing, but getting all those data will hinder my package performances?
The dataflow profiling feature use buffering to minimize the impact of I/O bottleneck generated by intensive disk writing. Some initial benchmarks shows that even under heavy load, performance impact is only by 3%. In exchange you have the possibility to deeply monitor the performance of your ETL. A good exchange IMHO.
I have a lot of already developed packages, some of them already deployed. Do I have to change something to use DTLoggedExec?
This is the amazing part of DTLoggedExec. No, you don't have to change a thing!
Since DTLoggedExec works directly with SSIS Engine, it is able to give you detailed loggind and dataflow profiling information even if your package was never developed to do so. DTLoggedExec is completely transparent to package developers and backward compatible.