Oh there's a huge difference. I already wrote about my experience in post #46 of this thread.
An OEM quickshifter is implemented within the engine control ECU. As a result, it knows, and can account for (not saying they all do, but they can):
- Throttle position
- RPM
- Road speed
- What gear you are in and what gear you are changing into
- Whether you have the clutch lever pulled in or not (disable quick shifting if so)
- Whether the clutch is slipping as a result of you pulling in the clutch lever (It knows what the RPM should be based on mathematics from the road speed and what gear you are in, and it knows what the RPM actually is)
- How strongly you are pulling on the shift lever (It's a strain gauge, which is an analog input, not a simple "if - then" switch that changes state at a fixed threshold)
In addition, because it is within the engine control ECU, it can detect the change in RPM that accounts for the shift being completed and resume power output the moment it detects that the shift has been completed, and/or it can detect the completion of the shift by seeing the sensor on the shift drum change state, as opposed to being a fixed preset time delay.
Aftermarket add-on boxes ... don't have most of this.
The OEM quickshifter on my beast works seamlessly. It just doesn't work with reverse shift pattern (yet - until I apply some mechanical engineering design to get around its design limitation).
I had to modify the add-on quickshifter on my other bike to disable it if the clutch lever is not completely out. Part-throttle driveability was intolerable otherwise.