I have myself 3 alchemists at 375 under my payroll, and I can say you the following:

1) Potion Mastery is the easiest of the three masteries to achieve if you have time and lack money. Transmutes if the other way around. When TBC started it was hard to get into either Black Morass (for elixir mastery) or Pot mastery (Botanica) with nice groups at all, so delivering some mats to get it done without any instancing involved sound appealing to me. Now everyone is at seventy and can easily get in, I'd save myself the trouble and go potions like said in previous posts.

2) If you ask me, best possible transmute to do with mastery is not primal might, by miles. On average you'll be paying slightly less for the 5 primals than the gold you'll get auctioning the might. If you are lucky, sure, will pay off; if no procs happen, you'll earn 10-30g depending on server prices for primals. If you want to go cheap, primal earth into water is the cheapest mat wise, and gives highest % returns (up to 5 times the gold invested, asuming a 4-5g price on primal earth). If you want to go higher, earthstorm diamond is your best bet: 2 primal earth, 2 primal water and 9 uncommon gems turned into a 150g metagem, with way better returns for your gold than primal might, even if NOT procs pop. If your team has a Jewelcrafter among your ranks (I strongly recommend you have at least one), prospecting will provide any uncommon gems needed, and you'll be able to cut the metagem into something even more expensive just with rep/trainer recipes. At least is what I normaly do.

3) However, unless you were extremely unlucky and got zero discoveries out of your trade, I suggest you stick with potion mastery or elixirs. Reason is twofold: Sooner of later, you'll get some flask discoveries that PROC with elixir mastery for very nice gold returns; if you are potion master, even without killer recipes, you'll make nice gold amounts out of mana and healing super potions. The second reason is the real deal: the more you craft potions or elixirs, the bigger your chances to see a proc happening. With transmutes, at 20h timer each try, you'll end up having horrible luck weeks with zero procs and lots of resources put to waste.

So, to sum up: if you wanna stick transmute mastery, go metagems over primal might, unless you can lay hand over a cheap suply of primals and/or can offer your transmute services in exchange of mats, trading the already crafter primal mights for mats (and keeping all procs for you later on). If you have money to invest, try getting some big deal recipes like haste or destruction potions and stick to potion mastery (advisable if your team plays on a mature server which has many guild doing end game raiding on Sunwell Plateau). If you want to go secure, get elixirs and craft as many flasks as you can. Trade mats for flasks, and keep crafting. You'll get more and more discoveries till you discover all available flasks, and you'll end up earning some nice gold.

My 2 cents and second post in here