I see the problem as this--
-The limit screws are adjusted improperly which is why you are having shifting issues, its not cable tension (just my guess)
The reason I say this is because if you are having to make the cable so incredibly tight to make it shift properly, then the issue isn't cable tension. It shouldn't need to be set very high at all for a front der. anyways. Usually just "taught" is a good tension, not "tight".
-The cable clamp is too tight!
Now both of these hinge on the issue that a) you have the cable routed properly in the clamp, and b) you are using the correct bottom bracket/crank combination.