Would the average sailor be able to tell the difference in performance of an aftermarket sail set vs. hobie?
It all depends on the sail....
There are some folks that are very happy with their aftermarket sails. I've see other after market sails that looked like bed sheets. Some even claim to be interchangeable between the different Hobie models - yea right.
I've personally never sailed anything other than Hobie sails, and even if I didn't race, I probably still wouldn't use anything but Hobie sails. There are many advantages of going with stock sails, but what it really boils down to (at least for me) is that with the Hobie sails, you know what you're getting- a high quality, long lasting set of sails with a proven level of performance that has been refined over several decades to work specifically with your boat. If you buy Hobie brand sails, you have guaranteed performance and longevity right out of the box. No sending back the sails for re-cuts. No dealing with flaky sail makers. No surprises. Just roll it out of the bag, hoist it up the mast, and you know it's going to work and work well for many years.