Are the speeds then in /hardware/boards.txt the maximum speeds for those boards?
No, these are the speed-setting for the bootloaders on the boards. If you change on of these, the upload speed used by the IDE will be different from (hardcoded) speed on the ATmega-bootloader which prevents successful uploading.
In your Sketches you are free to use any other speed you need. The maximum speed for the boards depends on so many different environmental and hardware factors that its hard to say what the highest possible speed might be.