1. From everything I've read, the Slow Stick can carry more then it's share of weight. You can actually use full size servos with it. In regards to say a HS 81, yes it will work, do you need it, it's more likely whether or not your going to move up to a stronger motor. Then the torque makes a difference, there will be a greater force against the elevator and rudder as you increase speed, and your servo won't slip. Speed always makes a difference in how fast the servo reacts.

2. As I understand it, Dual Conversion reads the signal twice, in order to sort out bad signals, while single conversion just takes the information received. In regards to postive or negative shift, Futaba and Hitec use negative shift while JR and Airtronics(Ibelieve) use postive shift. This has something to do with which side of the high or low frequency that the information is sent out on and read by the receiver.

Good luck.