You dont need any other mods to run a DCOE40, even Twin DCOE40's it just means that without the minimum of a small cam and header&exhaust you really wont get the benefit of the 40's though it will make a loverly difference...

As far as drivability, a properly setup DCOE40 is perfectly smooth and easy to use, I run Dual race prepped DCOE40's on my A15 and I think its even nicer to use than it was with the stock carb, I also get 15-17km/L when driving it normaly, driving it hard I get 11-12km/L which isnt bad at all when you consider its over 120hp!