I know bigger rims don't always mean bigger overall diammeters. On average most people go down 1" or 2" and you go down around 1 profile per inch to maintain a similar overall diammeter. Going up 6" on the wheel alone will be a problem if you're starting with low profile tyres already.
You must have different tyres to what we can get over here as I've never heard of a profile lower than 30 or 25. As far as I knew 15 and 20 profiles didn't exist. So any 20" tyres are going to be bigger than the 14s. If you have incredibly small profile tyres over there, then fair enough. Give me a link as I can't find any.
Maybe BetsyBio can tell us what tyres she has on the 20" wheels to clear up any confusion.
A 185/60R14 works out at 577mm diammeter
A 225/30/20 (smallest I can find) is 643mm and that is going to affect the gearing.
I fitted 215/35/16s to my 16" wheels, they are 556mm diammeter, smaller than quite alot of 14" tyres
and 30mm smaller than the tyres that came on my standard 15" wheels and they have helped my acceleration due to being a smaller diammeter.