You should track how you and your staff spend time for a week or so (a typical week). Then you should point out how much effort (FTEs) mundane support tasks are taking, and how much is left for system development, programming.
When you do that, do point out the extra penalty for efficiency due to constantly have to answer support request. (assuming it is inevitable)
Then list all requests for system improvements, what are the benefits of each when looking from a business perspective (bottom line impact). Do a rough estimate of development time, vs your limited capacity, and thus how long (if ever) the request pipeline will be cleared.
You should then lay three options for management: forsake a part of the $ benefits from improving the system, out source development or hire more staff. You can compare the cost of each alternative to make the decision of easier... (opportunity cost in the first alternative)