According to the manufacturer's website, the "hi-speed" (i.e. Water Injected) PSS does not need to be burped. All C350's after about #204 or so are equipped from the factory with the water injected / hi-speed shaft seal. All prior boats have been provided with a retrofit kit to install the water injection feature.
Here's a link to the PSS manufacturers Installation Instruction page:
http://www.shaftseal.com/pss-shaft-seal ... ction.html
and here is the direct quote of their instructions for "burping" the seal - and their notation that burping is not required on the "hi-speed" version:
11-b. Standard speed seals
When a boat with a watertight (P.S.S.) seal goes back in the water, there will be an air pocket trapped in the shaft log (stern tube). This air pocket must be vented so water can reach the face of the seal to help cool and lubricate it. To vent the air pocket, simply compress the bellow (push the carbon away from the stainless steel rotor with your hand) so that water fills the shaft log (stern tube). A small amount of water will enter the boat at this time and will stop as soon as you release the bellow, allowing the two faces to come back in contact.
This procedure should be done every time the boat goes back in the water and is not required with high speed seals.
Good Luck!