With no meta gem i would be going all out haste, no questions asked. Especially if you are doing more single target than cleaving, since from my own experience, mastery cant compare to haste single target before you can be around 65% ish. The cast times will simply be to long and if your overload or EotE doesn't proc, your dps will suffer alot!
Spam haste until you get LMG and then go for mastery. Rough baseline though, but it will yield great results. Only other advice would be to sim yourself.