To be honest, the best setup is the one you are used to. You could take the best WW2 fighter pilot and put him in the 'best' fighter now and he would be even worse that the old one because he wasn't used to it. Same with wow the interface is only a tool to help you, the real skill comes from the Brain addon which lots of people don't seam to have installed.
There are probably people who can outheal you with only the default ui and some mouseover macros because they have got used to that setup and just 'know' that 'person x' is going to get spam healed and therefore you don't waste your mana where as someone else has an addon to say that. Just because you cant do that doesn't mean that someone else can't. Using your brain and learning the right target for the right heal at the right time will improve your healing hundreds of percent more than a custom UI.
Basically my point is this arguement is fairly pointless, if you have used an custom UI stick with it don't bother swapping, same as the choice between bigwigs/DBM, when you get used to one system it just feels 'better'.