Are there any advantages (image quality wise) to upgrade the standard zoom on HPX500 to the latest Fujinon lens (HAc18x7.6)?

Fundamentally, which one is imposing the limiting factor (quality wise) on the video for the HPX500? The CCD or the lens?

I know if you compare HPX500 versus Sony EX-1 or EX-3 - the Sony is way much sharper because the imager is using a full raster 1920x1080 pixel.

Thanks.

Wading into very subjective waters here. If the lens on your HPX-500 has CAC circuitry then I don't think you will gain much in picture quality with another lens. The HPX-500 sensor is not going to pick up much more info from the new lens. If your lens is not CAC but some other HD lens then you "might" see more of a difference.
As far as the difference between EX1, EX3 and HPX 500 footage I like the quality coming out of the 500 and have used them together where the footage was cut together and the end results did not show the 500 to be lacking. The 2/3 inch sensor had some advantages over the 1/2 inch sensor as far as look was concerned and the sharpness advantage the EX series has in pixel structure and record was not a knockout blow by any means.
Biggest difference between cameras was ergonomics and media management issues and whether post was prepared for either P2 or XDCAM Ex.

I agree. Especially in 720p the two cameras will not be much different and the HPX-500 has a better look to my eyes.

The lens situation really requires you to see the images with both lenses side by side and make a judgement call. I think a better lens does improve the image just like a poor lens will degrade the image. Do you have a chance to try both lenses before making the decision?

I decided not to upgrade. I tested it - but, as you said, very little difference to warrant paying for a new lens. Thanks.