Yep, you can do that. We did it and got a table right next to the stage. Plus it wasn't nearly as crowded - Tier 1 was full, but Tier 2 and 3 only had a couple of tables filled.
Have not been to the Luau but going next trip using DDP. Have been to HDDR & really enjoyed it. But I like G-rated corny shows & it had us all laughing - both times we went even though the jokes were about the same.
Personally if I were staying at the POLY the ease of everyone being able to get to the luau would be enough for me to keep a ressie. It's in theme with your resort, you don't have to worry about 17 people traveling & you can have pre or post luau gatherings in your rooms, pool or bar together.