There are many different segments to the retail insurance broking industry, ie the 1) globals; 2) top 10 public; 3) top 100; 4) the SuperRegionals; 5) the Regionals; 6) single-location agencies; 7) small independents under $1M of commission....to identify just a few - all of them have different models, alignments, characteristics...and yes...metrics vary for each as much as cultures do.
So carving out data that applies to each segment that can be relied upon to most ably address your question is problematic at best.
If you are looking for guidelines to budget against, perform projections or establish goals for next year, or develop performance guidelines against - that's another dynamic as well. And, while taking into account industry data is always a good idea for those purposes, at the end of the day you should set the standard, the goals, and the metrics that will serve to create the ROI you have determined is the appropriate reward for your efforts, time, investment and sacrifices.....
We do have benchmark data here at Rainmaker - however, as I like to answer posts on the Insurance Journal 'sans-sales pitch' I would encourage you to start by looking at the financials for the public brokers - easy to find on the internet, check out their quarterlies and the Chairman's comments about different business units, lines of coverage, growth in certain markets and industries, etc., etc., and you'll at least get credible data which has the benefit of not only being large numbers (thus lowering standard deviation) but audited as well for accuracy. This should give you a good 'gut-check' and provide you with a starting point for determining your own corridors of performance for your agency.
Hope that helps