I can't think what criticism of the sail you are referring to
- these boats make excellent sailers and the sails are well cut, well sized for the boats, generate enough power (in the light winds for which they are designed) to be able to move the boats along at a perfectly acceptable pace, and allow the boats to be sailed upwind as well as downwind (unlike many/most aftermarket kayak sails which are really only for cross- or down-wind sailing).
Of course, on the basis that nothing is perfect, tweaks have been developed to improve sailing performance; such things as:
* Using blocks to ease friction in the mainsheet;
* Moving the sheeting point from its usual position on the rear padeye;
* Staying the mast to prevent mast bend/improve sail shape;
* Adding a jib;
* Adding roller reefing
* Adding outriggers for extra stability.
Some of these tweaks are "advisable"; others are very much non-essential and a decision to spend money on them would be very much down to personal preference.
Another factor is that some of the Hobie Mirage Drive kayaks are undoubtedly better sailers than others based primarily on hull shape. The best in my experience is the Adventure particularly with the optional daggerboard; I haven't sailed the Revo, Pro Angler or Sport but I have sailed the Outback and the doubles: their sailing performance is not as good as the Adventure.
In essence I would contend that there is nothing at all wrong with the performance of the stock standard sail straight out of the box on a stock standard Mirage Drive kayak and I strongly suspect that a Hobie Mirage Drive Kayak with the standard sail is the best off-the-shelf kayak sailing offering currently available on the market.
I have certainly had a huge amount of use and enjoyment out of my Hobie kayak sailing equipment (1xAdventure, 1xAI, 1xOutfitter, 1xAI sail, 2xkayak sails, 1x kayak jib, 1xstandard mast; 1x stayed mast; 2xdaggerboard)...