Willi Leopold's Malika uses massive outdoor levels - it is quite demanding on your system (the frame rates are quite low on old systems such as my P3 600, but then Black and White wasn't much less demanding), but it is very impressive and really shows that A5 is capable of nice looking and large outdoor areas (I am lucky enough to have tested it). And, considering that it will take you a while to complete your game - and thus by the time you finish, average specs will have gone up (a P3 600 such as mine is really ancient even now) - then so long as you are careful about your level building (see Willi's tutorial in User Contributions), a Black and White-style game built with A5 should be perfectly possible. (Not to mention the fact that JCL has mentioned, in passing, that A6 will have a faster kernel.)
Cheers,
Keith