The printer head doesn't move at full speed everywhere. Printers have "acceleration" and "jerk" specified in the firmware to control the head as it approaches a direction change or stop point. If you were to plot the speed of the head as it approaches a stop point, it would appear as a ramp function. Coast works more like a square wave function, where it just stop the extrusion and the residual pressure in the nozzle provides the extrusion force. It would be great if there was an enhanced coast feature that decelerated the extrusion rate for some specified distance as the printer head slowed down before stopping it completely. There would be some "tapering" coasting distance where the extrusion rate goes from 100% to XX% over X.XXmm, then it could have a traditional coasting distance where the extrusion is stopped fully if needed. I think this strategy could allow more refined tweaks on perimeters to get a seamless start/stop point. Right now, I always have either a slight over- or under-extrusion at my start/stop points and can never get it perfect.
Thanks for considering this feature,
Sean