My 5 minute time frame was for the length of your opener. I agree that people will form impressions of you much faster than that. A great book on this topic is Malcomb Gladwell's Blink.
My response to what do you first say to the audience is... nothing. I begin the first effect. After that effect, I do my introduction as part of my transition to my next effect. I let my performance speak for me. Simply put, I show rather than tell. A couple of examples:
1. Kids Show Operner #1 - Black Eyed Pea's "I Got a Feeling" begins playing, I display a deck of cards and begin "52 cards, 4 suits - hearts, clubs, diamonds and spades, 13 cards in each suit - 2 though 10, Jack, Queen, King and Ace." I put the cards down on the table and pick up a multicolored beachball. "I beachball, full of hot air. Let's have some fun" I throw it out into the audience. "If you have the beachball when the music stops, you have to make a choice...."
2. Adult Parlor Show Opener - I begin by tapping two 12 inch rings together three times (making a bell like sound) and handing one of the rings to a spectator on the left side. I then pick up another ring and tap it with the remaining ring three times (three more bell sounds) and hand it to a spectator to the right side. An orchestral version of Fireflies begins. I mime to the spectators to hold the rings up and inspect them. I then mime to the spectators to put their hands through the hole in the middle of the ring. I then begin my linking rings routine.
3. All Ages Show Opener - The show starts with me sitting on a wooden stool with my back to the audience. I am wearing black pants, French blue button down shirt, black blazer and a top hat. Upside Down by Jack Johnson begins playing. I slide off the stool, turn to the audience, remove the hat and do a slight bow bringing the hat to the side (and thereby showing it empty). I place the hat on the stool, brim up. I reach into my pocket and remove a black silk handkerchief, quickly show it to the left and then to the right and then vanish the silk as a cane appears. I place cane by the stool. I take a white silk out of my pocket and quickly show it left and then right. I show my left hand empty, stuff the white silk into my hand, grab the cane and (using it as a magic wand) tap hand to reveal first billiard ball. I put ball in the hat. I reach into the inside breast pocket of my jacket and remove a purse frame. I hold the purse frame in my left hand and show the purse frame as being empty. With a wave of my right hand and a tap of the cane, the second billiard ball appears. I put the second ball in the hat. I repeat the production with the third ball and put it in the hat. I try to repeat the production for the fourth ball, but it doesn't work because I waive my right hand but don’t tap my hand with the cane. I open my jacket and reach into my inside pocket and give an expression that the ball isn’t there. I tap my pocket with the cane and then reach into pocket and pull out ball and put into hat. This is done in a humorous way because the audience knows the ball was in my pocket all the time. I then go into my billiard balls routine.
Much like Christopher, these are suited to my style which very much emphasizes magic as being imaginative play and storytelling tied together by artistry.
I would argue that there is nothing you can say that speaks as clearly about who you are as a magician then performing your first effect.