Do you know how I would calculate how effective my GPU would be for different scrypt coins?
Is there a calculation with difficulty?
Go to http://www.whattomine.com/coins
There you can enter in your GPU's hashrate on various algorithms (coins) as well as your card's power usage and your electricity rate to get a ballpark figure of where you would end up with different coins.
That's exactly one of my questions...
So on scrypt, my hashrate will be the same across all coins?
On X11, my hashrate will be the same across all coins?
Yes, each coin that falls under a single algorithm would have pretty much the same hash-rate across all the coins using it. So for instance Ethereum (ETH), Ethereum Classic (ETC), Expanse (EXP) using Ethash would all use the same hash-rate figure. Where it gets tricky is they all have vastly different "Network" hashrates, so the actual amount of coins you will produce will vary. This is why the comparison sites like What-to-mine work so well.