I am not a Jewelcrafter, but I am able to see the obvious. Lets say that gemming a secondary stat has a higher benefit than gemming full primary stat (this is the case for many current specs). As a Jewelcrafter you are forced to gem 2x 320 primary stat to get the 320+ primary stat bonus from a crafting profession, but by doing so you are losing the possibility of gemming 320 secondary stat in those two gem spots.
Hence the performance cost of having Jewelcrafting as a profession (over any other non-BS crafting profession) is worth the loss of not having 2x 320 secondary stats, minus the gain of having 2x 160 primary stats.
Blizzard should just drop all this "uniqueness" of gear benefit of a profession, and add more uniqueness in what a profession is able to craft in terms of vanity crap/tradeable performance increases.
Every profession should grant 320+ primary stat. No secondary stats, no procs, no special enchants, or "on-use" effects that can be tied into cooldowns. Blacksmiths should only be allowed to put in (rare-type) primary stat gems into their extra sockets, and Jewelcrafters should be able to put a 160+ primary bonus on two gems of choice.
Anything else is flawed by design.