IVal too large means that your overall gear ratio is much too small.
The 1200 limit is intentional to prevent issues with performing low speed movements, and to allow the acceleration/deceleration tables to work correctly. Values larger that this can result in unintended overflows in some calculations.
Additionally if you are on the high end of IVal, it means the resolution of the mount is going to be very poor - the DEC axis will move about 3 arcseconds each microstep.