Aqui vai um material da UN-Habitat que pode ajudar nessa argumentação:

http://mirror.unhabitat.org/pmss/(X(1)S(monoxp5nlpxxrh4ejj2b5k4h))/listItemDetails.aspx?publicationID=3513

“One critical ?nding is that the City Prosperity Index is
higher than 0.800 (compared to the maximum, 1) among
cities that enjoy high street connectivity, good infrastructure
development, good environmental sustainability, high
productivity and quality of life, and also high levels of equity
and social inclusion. In other terms, these cities do well in
all components of prosperity, including street connectivity.
Provision of basic services (water, sanitation and drainage
facilities) is quasi-universal in these cities. With good street
connectivity, these cities also enjoy high productivity with
optimal commuting time to work and other services. They
have a high productivity index associated with reduced traf?c
congestion and improved walkability through better street
connectivity.”

Desconheço estudos que são assim tão específicos. Há vários estudos da Sintaxe Espacial que mostram que áreas de centralidades locais acontecem em lugares em que há o que eles chamam de “intensificação da malha”, mas medidas específicas não me lembro de ter visto. Pessoalmente, também acredito que 300m seja um valor excessivamente alto.