From your numbers above, it seems your staffers are taking, on average, about 80 calls per day. Based on your current talk time, that's a reasonable number of calls, and represents about one call every six minutes per agent. Depending on the wrap requirements, that's also a reasonable level of activity, with just a bit of breathing room between calls but not a great deal of downtime.
However, I would also say this call center is operating very near capacity. With such a small staff, you probably have a very tough day if even one person is out sick. If someone is on vacation and someone else calls in sick, I imagine your abandon rate and hold times skyrocket. Also, depending on your daily call distribution, I expect lunch times and peak periods are extremely busy.
You might consider making the case that having little or no "wiggle room" in terms of staffing can be problematic. Even finding the time for a team meeting or having your team assist you with updating documentation, etc. can be difficult with a busy call center.
Another approach for management might be to indicate that a 2 1/2 minute call is quite short. There's little time for building customer rapport, cross- or up-selling, or offering that extra, "Is there anything else I can help you with today?"
Are these agents also responsible for other contacts, such as emails or online chat?
I don't think you will find such a thing as an "industry standard" metric for call centers in general, but I recommend seeing what you can find for other banking-specific call centers. I currently manage a technical support/telephone help desk call center, and our average call time is closer to seven minutes, with rather extensive follow-up and wrap requirements. I am pleased when our agents close 20 tickets per day (though we have peaked at far more). Another call center I manage is almost exclusively Tier 1 support with average call durations of about 4 minutes. The wrap requirements are minimal. That group averages 40-50 tickets closed per day. That's an example of why it's very difficult to find worthwhile benchmarks outside a particular industry.
Another suggestion would be to combine your request for additional resources with descriptions of the measures you are taking (or have already taken) to improve efficiency and effectiveness, such as upgrading computer equipment to speed up screen loads, giving agents access to better tools or reference materials, improving escalation procedures, etc. That way you can make the case that you are getting the most out of the staff you currently have.
Best of luck to you!