Re: stiffen sails - how often?
It's certainly not a "scam" per se, but I have never seen a good ROI posted on that process in thirty years. Might make 'em look crispier for a short time, but it's the breakdown in the cloth that hurts them.
If the cloth still passes inspection, have the sails recut. I have done that on a couple of boats and it really made a good difference in adding back performance and reducing heel.