Hello.
In our testing here, we found the boards can provide about 1.5A/phase continuously without any supplementary cooling. The amount of extra current the board can provide depends on the cooling system, but it is not very practical to get 2.5A/phase out (also, the current sensing resistors we use do not allow settings above 2.2A). Unfortunately, we do not have any other boards we would expect to work well at that sort of current.
Also, the absolute maximum voltage for the DRV8825 board is 45V. That voltage is not related to the current limit. If you are interested, there is an explanation of how current limiting drivers like the DRV8825 can be used with higher supply voltages on the FAQ for the driver. You might also be interested in this thread on our forum, which discusses how the supply voltage affects the maximum step rate.
-Nathan