I'm considering purchasing an i9s with sail kit. Would like to hear from anyone who has sailed one of these to find out how well they sail upwind as well as with the wind & also, in general, how stable they are when being sailed.
The 1-9 is a fun little sailer in light to moderate winds. You should have the Large sailing rudder. This short boat tends to have a heavy weather helm (tendency to turn into the wind) so the larger rudder is essential. It's also noticeably helpful to rake (tilt) the sail forward with the forestay to help balance the helm as shown in the picture below:
Upwind sailing is not as efficient as with most sailboats, but can be improved with Turbofins (which act as a larger centerboard). Actually, upwind performance is pretty darned good if you don't mind pedaling while sailing (doesn't seem to cause any additional side slip). The mast will yield with sudden gusts because of the flexible fabric. IMO the stability is good, especially for a kayak -- the i-9 has a broad beam.
Welcome to the forum!