Your approach is correct.
Performance throughput?
IMHO no need to worry – JPA 2.1 will use an equivalent mechanism. But make sure to write a realistic test to be on the safe side.
What happens when there are multiple logged in users updating entities
concurrently in their own scopes?
All (non-dependend-scoped) bean references are proxied internally. The underlaying CDI-implementation has to guarantee the correct resolution.