Technically, if the tilt rotor flies forwards with vertical rotors, the lift is produced by the wings and forward thrust by the rotors. So let's regard the rotor thrust, whether in horizontal or vertical rotor position. Tilt of the rotor does not affect the amount of thrust produced. The following factors do:
- Rotor speed. This would remain pretty constant over the tilt range, with tip speed as close as possible to drag divergence speed. At top speed with vertical rotors, rotor speed must be lower than in the hover because of the velocity triangle tip speed = rotor speed + air speed.
- Blade Angle of Attack. A function of blade angle and of airstream inflow.
- Airstream inflow. With the rotors in a vertical position and flying forwards, air flows into the rotor disk and requires less acceleration to provide the same amount of thrust. Or: a given thrust requires less engine power in forward flight than in vertical take-off.
Therefore, for a given amount of engine power the vertical rotor at speed would be able to provide the highest thrust - but depending on the max. speed of the craft, rotor speed must be reduced, which also reduces the ability of the rotor to convert power into thrust.
The picture above from Leishman, also used in this answer, shows the amount of engine power required vs. airspeed for helicopters. Note that a helicopter rotor must always provide more thrust than its weight. In order to gain airspeed, the main rotor is tilted as well, and the effect or airflow at higher speed can be seen from the reduced amount of induced power.
For a tiltrotor, the rotors will be dimensioned for vertical take-off: they must provide thrust > weight in this situation. In horizontal flight the wings provide the vertical force for weight compensation, and the rotors only need to overcome drag. Which increases quadratically with speed, so at top speed thrust will be the same as TO weight again.