It is your CV that needs to be doing the leg work and if your cover letter is over-egged it simply means that you have not put the effort in to write a focused and industry-specific CV. Employers will recognise this straight away. This is essential in a challenging labour market.
You must also remember that in the recruitment proces your CV may even be separated from your CV and if you’ve poured all of your energy into your cover letter your CV will look very weak on its own.
A weak CV that needs a cover letter to scream all of the highlights is a CV that needs a complete overhaul.
No doubt you are using your cover letter to highlight areas of your CV that you think that particular employer would like to see ; that’s great but you may be better off adding a Skills Profile to your CV or looking at better ways of using your Personal Profile to position yourself deftly to the targeted role.
It goes back to the same old adage: if you’re CV isn’t doing the selling then you’re off target from where you need to be. Just as with the job interview question focus, aim your CV to the bullseye and nowhere else.