Revised:
There are many connection options supported in the private level, but the connection string and Client object should support only two for consistency with other drivers:

• maxPoolSize - maximum number of (non-monitor) simultaneous connections per host
• maxIdleTimeMS - time after which idle connections are closed; if set, then after an activity spike up to maxPoolSize, idle connections will drain away.

Original:

I don't see how connection pool options in the connection string (which appear to be non-standard anyway) are wired up to affect the actual connection pooling.

We should investigate to confirm if this is working or broken and, if broken, develop a plan to fix it.

We don't necessarily need to fix problems for the alpha release, but we do need to be able to describe known issues to address in the beta.