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hit still requires some work to get to 315
Well, a little maybe, but in a short while raid gear itemization will make the poison cap trivial too, maybe not at entry level, but certainly by Uldaur I'd imagine. The difference now is that hit over the poison cap is definately a worse choice than Agi/AP, whereas, pre WF change, a case could be made for capping yourself (that is, capping hit could under certain circumstances provide dps competitive with other gear choices, say Agi or Haste stacking). The hard hit cap is now, what...? 700 or something? The gem you originally mentioned might well be useful situationally, if getting a new piece of gear with a drop in Exp/Hit puts you slightly under either cap, but flat gemming of hit/exp gems would require a really weird gear set up.
Plugging your gear and spec into a spreadsheet can be tedious, but you only actually have to do it once; after that, you're just fiddling around fine tuning it. You then use a loot mod or a gear list like Shadowpanther to determine where your next upgrades lie (if they are in fact upgrades), so you're ready to bid or roll if they drop.