The first Nestorians only arrived in China during the early Tang, if I recall (which is around 600 AD). If they arrive earlier, and they somehow mass-convert China to this new faith (almost ASB to say the least, but stranger things have happened), then you'd get an extra 20% or something.
Greater economic links with India could help in creating more southern Indian Christian states, which could eventually help in making India Christian. Persia, again, would require some divine intervention to convert the Sassanids and make them Christian too. Even with those, though, I think the populations of sub-Saharan Africa, Mesoamerica, Australasia and Siberia would still probably make up more than 10% of the world.
I guess it could have helped if Christianity developed its 'just war' clause earlier and specifically declared 'converting the heathen' as a casus belli in itself.