To the OP: I just dined at Lotus of Siam in Vegas and thought it was fairly spicy, but I was in a big group (mixed Asian and "other") so I think it got toned down a bit for us. Great place to eat

Other utterly authentic Thai places that don't hold back on the Spice level are

Spoon Thai in Chicago - ask for the secret Thai menu where they have hundreds of truly authentic Thai dishes like banana blossom salad, catfish curry custard, pork neck salad, raw shrimp w/ chili peppers & fish sauce. That dish really was incredible, but super super spicy, and I'm Korean and accustomed to spice.

Sripriphai in Queens, NY - pretty well known place amongst food lovers in NYC. They don't hold back on the spice either.

Neither of these hold a candle though to Spicy & Tasty in Flushing Queens which is Chinese Sichuan cooking. YEEEOUCH.