What do you use the RBF for? For creating/interpolating curves, transforms, images, models, …?
There are single-line utility functions to use numpy arrays directly in MRML nodes. Some functions allow you to access MRML node content as a readable/writable numpy array, so that if you update the numpy array then the displayed data update live.
There are also many modeling and interpolation tools in VTK that are much more sophisticated and faster than similar functions in scipy.