How much shielding is "enough" depends on how much interfering field is present and what interference you are willing to tolerate.
The main problem you will have with CAT5 cable is that is has the wrong impedance and is likely to create ghost images or blur edges horizontally, depending on how long a splice you make. You might be able to remedy those problems by changing the source or load termination, but using the right kind of coax would be simpler.