Is it worth the risk substituting a lookalike for the real thing?
The problem with counterfeit parts often lies in the material, not the dimensional quality. If the manufacturer substituted inferior resin (regrind) for the injection molds, you may not know until it fails.
So like anything, the risk comes down to likelihood and consequence of failure. Likelihood is hard to determine. Consequence is something you can determine.
If it leaks or cracks, are you okay with blackwater leaking where the pump is located?
If the impeller spins on the shaft, is it easy to get to to replace?
Etc...
If the answers are Yes, then the consequences are acceptable.
If the answers are no, then reconsider.
I feel that the counterfeit market is really aimed at people in emerging economies, where people are more accepting of inferior quality, as the reward of ownership of something otherwise unaffordable outweighs the risk.
For the record, I might consider the pump. When I replumbed my holding tank I made sure my macerator is fairly easy to get to, and it's leak path is direct to a bilge pocket. I also believe that Jabsco products are way overpriced for what they are, and rarely improve their product line. Just another revenue center for Xylem.