Why is parameterized SQL generated by NHibernate just as fast as a stored procedure?

It is just as fast if the query is the same each time. Sql Server 2005 caches query plans at the level of each statement in a batch, regardless of where the SQL comes from.

The long-term difference might be that stored procedures are many, many times easier for a DBA to manage and tune, whereas hundreds of different queries that have to be gleaned from profiler traces are a nightmare.

Measure it.

Really, any discussion on this topic is probably futile until you've measured it.