Perhaps, if you pardon the pun, I could through a little more fuel on the fire by suggesting that if you so not adhere to the manufacturers fire drill you are opening yourself up to a liability action.
I know it makes common sense to fire off both bottles straightaway in this situation but if the fire drill says wait 30 seconds before firing the second bottle then perhaps we should do so!
As regards pax evac then as has already been said it all depends on the situation. Some airports in the world do not have a huge amount of fire-cover and I would not like to bet too much on how long it might take for an appliance to reach the scene.
The first step in the solution of any problem is the acquisition of the correct information and, in the case of a fire, seconds may count. However, the consequences of a hasty decision to evacuate the pax may also be costly. It's just one of those decisions which captains are paid to make anf then face the music (and hopefully dance!).