8% throughput penalty using use keys [] vs scalar use keys

Q1 uses a request of the form

select * from default use keys [$1]

If I change the request to ... use keys $1, on my little performance testbed throughput improves from 15500 req/sec to 16700 (a good 8%).
While we should change Q1 on perfrunner, we should also make scanning the array faster in the key scan operator.