The limit for this particular circuit is the current handling capacity and power dissipation ability of the internal switch in the LM2585. Due to the design of these chips, they're simple to use, but you can't really add an external switch to them to boost their capacity.
An output of 2.5A @ 24V is 60W, so you're getting into non-trivial boost converter sizing here. If you manage to get 80% efficiency, you're talking input currents of 6.25A @ 12V - ie. 75W, with 15W lost in the converter, so you probably want to aim for more efficient than that, which will normally mean synchronous rectification etc. Good, modern converters can reach 95% or so efficiency.
Head over to
http://www.ti.com/ and have a look at their Webbench software - you can plug in your input voltage range, desired output voltage and current, and it will give you a number of design options which you can then refine.