Well outdoor levels have low FPS
The problem with the A6 is that is has a normal culling system,
called BSP tree. (Binary Spave Partition)
The BSP tree system is the fastest and the most effective, especially for indoor levels, but has the disadvantage that the BSP tree must be precalculated by the level editor.So this requires compilation of the level every-time one makes a change to his level
So basicly this system is used for indoor levels.
The second culling system that the A6 engine, saddely doesn't include, is the Portal or Octree which is very effective when using the outdoor levels.

Now I'll give you an introduction to LOD system:
The LOD system is know as Level Of Detail system which helps the engine inceasing the FPS in the outdoor level.The LOD system automatically switches to 'simpler' shapes of objects when they are far away from the camera, thus reducing the overall number of polygon drawn per second.As soon the shapes or object come closer it automatically starts showing detailed stuff instantly.

What can also reduce the FPS is using low poly models and a lack of sprites !!
This can make a huge difference !!
Also, a new plugin on the market named Sphere Engine made by Matt_Aufderheide, is an exellent plugin when using outdoor levels.
I have even another suggestion for you, just make sure when compiling that the
visibilty calculation and the light calculation is set to low like here:
http://img226.imageshack.us/my.php?image=compiler5xw.jpg

Well maybe abit huge to swallow at one piece but this is just the plain truth
Not that I don't love the A6 engine, it's simply the BEST game engine on the market !!

Cheers

Frazzle