Your room conditions aren't going to change the light coming from your projector. If the lights are off and you are only getting 8 foot lamberts on your screen, that is the most you are going to get.
Turning on the lights or white walls/ceiling might raise your foot lamberts as more light is hitting the screen but that is not the light you want. The white walls or ambient light is what hurts ANSI contrast. A bat cave will not raise the foot lamberts but will help ANSI contrast.
Are you using a 100ire window or field pattern for the measurement? It should be a field for this measurement. That can cause a bit of a discrepenancy, depending on the meter you are using.
How big is your screen? What gain? What projector?