In my opinion the gridseed will kick gpu's from Scrypt, same as before the Asics kicked the Gpu from sha coins. I've said that before and some users said that will never happen, well, I'm not an expert but I think this is already happening, slowly yes, but just wait a bit the next gridseed generation.
Gpu's will need to move, I did it already, I just mine Scrypt-N coins, waiting to maybe wafflepool or another one makes a multipool of Scrypt-N coins.
I doubt it will be gridseeds that cause the end of profitable GPU scrypt mining. They aren't better enough to do that. However, when we see 100MH/s ASIC equipment drop from Alpha-T, KnC, Fibb (if they are legit), and whoever else popups up, GPUs will need to be on another algorithm. My bet is on Alpha-T shipping first, and scrypt GPUs will be unprofitable vs power within the following month or two.
I have already moved a chunk of my GPU farm to vertcoin (nscrypt), in anticipation.
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Would love to see a 'vertcoin-like' coin switching pool. EXECoin, Vertcoin, SpainCoin... already enough to be switching on.