Energy: a mechanical seal consumes 7% a braided packing
Water saving: a double mechanical seal with API plan 53 can consume 10 gallons a year, a braided packing thousands of gallons of water.
The mechanical seal for pulp with API plan costs about $ 20,000 for 2 "shaft.
A mechanical seal requires skilled labor.

It is true that a dual mechanical seal set up with a plan 53 will consume a minimal amount of water. Althoug some systems use large amounts of water, packing can be set up with low flush rates, as low as 5 ml/minute.
You mention the figure of seals consuming "7% a braided packing". If this refers to power consumption, I would be interested in finding out how this was determined. It turns out that a recent study was commisioned by the European Sealing Association and the Fluid Sealing Association from an independent test lab to find out the actual power consumption of braided packing compared to mechanical seals. Although more testing is planned, results from the comparison tests so far show that packing power consumption is actually equal or less that that of seals. The results were unexpected but conducted on the very same test setup.

I think mechanical seals and packing are not the right approach, with the start-up and shut down procedure the expeller design pumps make more sense. Dollars and cents wise, especially in a process very little shut downs.