What are the best scorecards/metrics to use to in benchmarking my investment managers?

In order to do so:

1) Identify the objectives in using an outside investment manager:
Improve yield?

Supplement internal staff in portfolio management, accounting, tracking and the like

2) Define liquidity horizon: daily, weekly, monthly, 90 days? Blend?

Once the above is done we can derive the appropriate benchmark. Until then just guessing.