HDMI.org say that all sub 5m cables built to Spec ought to perform equally as well as each other.
They do also say that differences in the quality of the HDMI Transmitter and Receiver chips used in Source, Switch and Display gear can have an impact on signal quality and in some cases the quality of the cable can prove beneficial - especially if the Source has a poor quality Transmitter chip installed.
Over 5m (and especially when you get over 10m with 1080p signals) the quality of the cable can be a deciding factor in having a working connection or not.
In terms of 'enhancing' the video signal with an HDMI cable I'd ask any vendor to loan you any cable which is claimed will enhance your image quality and test it against your current cable.
According to HDMI.org all 'built to spec' sub 5m cables will work equally well.
Once you go over 5m and venture into 10m+ territory its a different matter - some cables will work perfectly in some systems and not at all in others, some cables will work well at resolutions up to and including 720p/1080i in a system but wont work at 1080p in the same system and some cables will pretty much work in all systems at all resolutions.
In theory once you get a working system there ought not to be a difference in the image you see using various cables - though other folk dispute this and say they do see differences.