Stadium seating should not pose a problem for the HP. Take a look at the graphic labeled "Retro-Reflective" in post # 1 of the High Power thread. The audience heads need to be under the light path shown in dark yellow. Now imagine that graphic showing two rows of seating, the back row slightly elevated. Both rows of the audience can still receive the same gain by adjusting the projector & screen height. It's not critical to ensure that both rows are identical distances under the light path. If one row of heads is 18" under the beam while the other row is 24" underneath, the other row will have slightly less gain. No biggie. Viewers in both rows should still enjoy a significantly brighter image than they would on a matte white.
I'd recommend that you request samples of HP, matte white, and whatever other fabrics interest you before you pull the trigger on a new screen. Seeing the differences for yourself should help with your decision. Da-Lite samples are typically ~ 7". They will send out larger sample sizes on request however. Ask for an 8" x 10" or there about to better judge the material.