I was thinking "how can I save soldering 32 resistors?" and thought of doing what you did - a constant current sink. But I decided against it on the grounds that the current being sunk would be shared, and the more LEDs that lit up, the more current would need to be sunk, and that the results wouldn't be even. Your report appears to confirm that.
Perhaps you can clarify how you are managing the current. In the absence of resistors are you going for a constant current sink, or relying on PWM to limit it?
The most current management I'm doing is switching it with the MOSFET which can take up to 9A. Since the current is not constant, I can't really use a constant current sink. I may be lighting 1 LED or 64 LED's and ideally the current should increase proportionally based on the number of LEDs lit. I've done some PWM to flash 8 LED's at a time and that helps achieve uniform brightness (at the expense of maximum brightness). If I can't increase the current then I'll probably settle for a solution like this.