SJMazzella3380 said:

I don't think this is covered either in the rulebook nor the FAQ. I think you should submit a Rule question to FFG about it (see link at the bottom of the webpage). 

Personally I would convert the two surplus crits into normal wounds, i.e. the attack would do 2 critical and 2 normal wounds.

The number of crits becomes the new minimum damage that gets through soak (it normally is one), but they go through as normal damage. So if you do 8 damage with 5 crits against 6 total soak (armor+toughness), then you would do 3 normal wounds and 2 critical wounds. 

Doc, the Weasel said:

Have you found that rule in the book/FAQ somewhere? Or is it your take on it? Just wondering :)

Here is the reply that I got from FFG

Stephen,

I was going to post that answer, but the response from FFG beat me to it. The onyl time criticals get turned into normal wounds is if the total number of wounds inflicted (including criticals) is less that the target's soak.  Otherwise, you only do damage equal to the damage actually dealt after soak. Those will be criticals if you have enough criticals scored, but you don't 'gain' any additional damage for having rolled extra criticals.

As was mentioned/alluded to in Dan Clark's response, the dice results in WFRP are "optimal" with a balance of successes and boons.  Just like rolling too many successes doesn't help (unless you use the optional rule to convert extra successes into additional damage), too many boons won't necessarily help either.  You generally want 3 successes and 4 (or so) boons for a "best" roll.  Most actions don't have a success line requiring over 3 successes. Boons are a bit better for rolling more, since you have several options to spend boons on (universal two to restore a fatigue/stress, 2-4 to activate weapon's critical, 1-2 to trigger the action's boon line).