Haven't there been a bunch of studies about this sort of thing? Lyrics aren't relevant to someone who doesn't understand language. Voice is just another instrument. Syllables are just notes.
Classical music is frequently cited as beneficial to learning and it has no lyrics. Nobody has ever suggested rap music is intellectually stimulating in any context, and they use a lot of words.
Even on rats in mazes, blasting them with Metallica seems to result in worse performance than Mozart. Myself, I can't work while listening to rap, rock or metal. Techno or classical only. Inducing a state of aggression/heightened arousal never leads to cogent thought (I can't work to Lords of Acid either!). There's a reason generals lead from afar.
I suspect music exposure may be related to autism in children, as a product of the spectrum of stimulation they're exposed to-- too much overall (urban life), too much of one type (war, domestic violence), or too little of any ("frigid mother"). The classical arts (a form of expression) are another thing that dropped off as diagnoses rose. We do not value art like we used to. Who owns a piano anymore?