Mycetic spores are the Tyranid version of a drop pod. The Tyranids use them to land on the planet. The phage cells aren't like normal phages. They are like tiny Tyranid workers inside the body of each Tyranid, the same way Tyranids are workers in their hive, or in the bioship they are spawned in.but are the phage cells the same thing as the Mycetic Spores
Tyranid spores probably wouldn't cause the Orks to reproduce like mad. Plants, fungi etc will explode in growth after the Tyranids have mutated them. Often the huge numbers of plants about have already been absorbed in the Hive fleet and what is seen are their mutated, Tyranid-infected versions. Also what is often seen as an explosion of local plant-life isn't. It is Tyranid life only it's in plant-form.
If Tyranid spores were to speed up Ork reproduction then they would have a very strong effect on any Orks produced, such as hybrids forms or some sort of slave-race to be consumed. There are plenty of descriptions of humans becoming very ill because of the spores Tyranids release. Orks could become ill too but they are tougher and so less likely to get ill.