Double-blind studies seem to suggest that it's around 5%, though it's just an approximate and on dosages of 500-600mg/wk and it varies from study to study depending on the methodologies of determining it. Beyond those dosages, dunno.
Keep in mind, however, that "aggressiveness" is not the same as anger and/or irritability as the media would like to suggest. For example, you can feel "aggressive" when you're pounding your girlfriend or in the gym... but that doesn't mean you're actually angry at her or the weights.
In myself at least, I certainly feel more aggressive in both the mentioned contexts, however my overall mood has improved significantly and I am much more patient. It takes a lot to piss me off now.
Also keep in mind that the "roid rage" issues that you often hear about could just as easily be a result of other side effects of drugs such as estrogen elevation (which can be controlled through an AI + bloodwork), extra sweating/higher blood pressure or insomnia (though that's an issue with tren, not test) or even the introduction of other recreational drugs as is commonly associated with AAS users.
Placebo is probably the most common one by far, however. People like to have an excuse for their lack of self control.