Playing Alien Swarm on Brutal is nothing short of hilarious, because it is so excessively difficult. Is there somewhere (steam forums or elsewhere) that has a list of the stats of the aliens when played on Brutal?
A fully manned brutal mission under default settings features a swarm that has 2.6 times as much health as it has under a fully manned normal game. The damage dealt is also scaled by 2.6 times.
Additonally both the swarm and the players are faster, although I don't think they're any faster than they are in Insane (that's 20% faster for aliens and 4.8% faster for marines).
I have only found a spreadsheet that list players, classes and aliens data.
Another possibility is to take a look at the source code, but I do not have it here at the moment, if I find some info later in the code I will edit my answer.