Ok I found the problem. The Marlin version which the official firmware is based on is a bit different. In our current version, the MAX_POS redefines HOME_POS because MANUAL_*_HOME_POS is never actually used unless the previous
#define MANUAL_HOME_POSITIONS
is defined (which is not in our case). This was not happening in the older version of the firmware. Actually this is a harmless bug in our firmware: we are defining the MANUAL_HOME_POS values but not enabling them.
Now, I still don't understand why one would want to set the MAX_POS to 250 instead of 200 (maybe it is really for "manual" homing, where you travel far in X and Y, in which case Ian is correct, the firmware might stop the movement before the endstop is reached, but nobody really uses that anymore), but defining MANUAL_HOME_POSITIONS should make the new firmware work with "Solidoodle" values.
Adrian, if you have some time could you please check this, if it's not too much trouble? Thanks!
However, I suggest keeping our firmware as it is, with the correct limits, as I see no reason to change it.