Proguard is a shrinker and optimizer that includes a name obfuscation feature intended to further reduce the size of an application. Name obfuscation provides a very minimal level of protection, and other security features are required for adequate coverage.
With any changes to an application, whether it be optimizing or obfuscation or adding features and functionality the onus is on benchmarking the entire application before and after the changes. Profilers can be used to augment such an effort.
I hope this helps, and have a great weekend!
Jesse