PucP,
Vitreous Enamelled bowls are offered by several manufacturers of vertical turbine pumps (VTP), including Flowserve. There is a claim for higher efficiency due to the smooth passageways (reduced friction losses in the pump casing) in the pump diffuser.
I believe Vitreous Enamel is similiar to glass (the inside surface of the casing is coated and then baked in an oven), and is not particularly good for abrasive applications. However, since this type of pump (VTP) is particularly well suited to agricultural applications, vitreous enamel is apparently resistant to relatively small concentrations of sand. Sand is not a particularly abrasive material.
In abrasive applications, the enamel will fail quickly resulting in reduced efficiency, even lower efficiencies than standard materials would provide.
Although VTP's and Submersible "Deep Well" pumps are virtually of the same hydraulic design, the submersible pump industry has never bought into the idea of applying bowl coatings to improve efficiency and maintainability. I suspect that this is partially due to the difficulty in proving the advantage of Vitreous Enamel coating.