The optimum runner length will be spec dependant.
Generally long runner lean more towards a low rpm tune and short runner to high.
With very long runners ideally you would want a very good design,good material coupled with a correctly sized turbine housing(or full turbo for that matter) to ensure good gas speed is kept aswell as good gas temp.
Materials such as inconnel for the manifold and twin scroll turbo's are starting to become more common in.....road going cars. Its bloody expensive though.