Would the line-item veto power give the President too much power?
We must first realize that the President cannot be given a line-item veto unless by Constitutional amendment. The Congress did give the president this power in the Clinton Administration, but the law was struck down by the Supreme Court in 1998. The Court said it violated the idea of separation of powers because it gave the president power to change laws that had been passed by Congress. It is exceedingly unlikely that such an amendment will ever be passed.
As to your question, the argument could go either way. There are points to be made for both sides.
From one perspective, the line-item veto would not give the president too much power. The reason for this is that Congress would still have the power to override the vetoes. In this way, the line-item veto would not be all that different from a regular veto. It would not swing the power towards the president in any substantial way.
From the other perspective, the line-item veto is much different from the regular veto. From this perspective, it would allow the president too much power because it would allow him or her to pick specific parts of legislation to veto. This would allow the president to do things like destroying bargains that had been made with Congress. For example, imagine that Congress and the President today passed a compromise with tax cuts and spending cuts. Then imagine if President Obama were to veto some or all of the spending cuts. The Republicans would not be able to override and the law would be fundamentally changed.