As long as you can get the edges of the torn area to match back together again, or at least come close you should be able to do it. Tape it on both sides if you can so there's more grip from the tape. On a matte screen, if you work carefully the repair will be hardly noticible unless you are looking for it.
I have done this on three separate occasions:
One time when some idiot flicked a lit cigarette at the screen and made a burn mark about 2 inches long and a 1/2 inch wide. I got up on a ladder and taped over it carefully. Unless you were standing right, close to the screen you couldn't tell it was there. Even then, the only way you could tell was because there was an area where there weren't any perforations.
Another time was when some usher got fired and tried to get revenge. He and a couple of his buddies took a knife and cut a 3 foot gash near the bottom of the screen. I had to use heavy white thread to "knit" the gash back together before I taped over it but once it was done it was almost impossible to notice if you were sitting three rows back.
I can't guarantee that this will work in all cases but I've had good luck with it. When you are faced with the choice of having a huge gash in your screen or else trying to repair it you probably don't have anything to lose. You're probably going to have to eventually replace the screen anyway. (All three of the ones I did were eventually replaced.)
At least this will buy you some time till you can sit down and think about what you want to do.
Sounds like you have that part well in hand!The screen I have in my auditorium now is a 1.5 gain. (Hurley Super-Glo) It's pretty nice. You get even light and the customers sitting at the edges can see pretty well. This is a "Continental Style" theatre with the aisles down the sides instead of the middle, so we have a pretty wide dispersion of people. I don't know how far off center your people will sit. In my estimation, it won't be too bad.
There will probably will be some fall-off toward the edges of the viewing area but you have to think in terms of trade off. Will the increase due to the gain be worth it for the decrease in the width of the viewing area. I would think that if you have a "festival" type crowd where people just pull up blankets and lawn chairs wherever they want, nobody will notice. People will seem to just, for no particular reason, all of a sudden pack themselves more toward the center of the field.