Hi,

Indeed you can use the two positions for powerful and less powerful engines. You need to recalibrate, or you can save the two configurations and restore the one you need.

In general, the load cells are very linear and noise is easy to filter out. But any small amount of drift or inaccuracy in calibration etc will cause a larger error percentwise error on smaller engines, so you are right to be thinking about this. 

For the load valve, there is the Land & Sea option, which is over priced in my opinion ($1100 or so), but it is plug and play with YourDyno and includes a stepper and stepper controller. The only other alternative I know about is this one: https://valvesolutions.com/product/v-port-control-ball-valve-series-e/. You need to remove the motor controller and add a (much faster) stepper. I have not seen this in action, but from the specs it looks good.