Michel,
Motor will definitely slow down under load. By how much, largely depends on the complete system and load (including inertia, electrical power input, motor efficiency etc). You can establish this relationship by running load tests with the drive. If the relationship is linear across all the variations (like input voltage, loads) you can drive is with x duty cycle for y output.
From DRV10983, You can read the speed directly (on FG pin or I2C register). The current is measured inside the device (for algorithm and protection) but is not given out to user.
Thanks,
Jasraj