Rafael,
Your 20" wheel bike is just like any 27" wheel bike. Don't let the wheel size confuse you regarding components. Only factor is gearing. A small wheel makes a given gear combo lower.
I recently replaced the Sora rear derailleur (same one as in the above pics, RD-3300) with a Deore long-cage. It has the chain capacity to accomodate the 11-34 cog and 52/40 front.
Back to my original question:
what do you want to do?
My rear-derailleur upgrade had one use: accomodate the Megarange rear cassette and front crank-set upgrade. The Sora worked fine but didn't have the chain capacity or the ability to use a 34T-rear cog.
XTR would "feel nice" I'm sure. But it also uses 4-bolt pattern MTB chain rings, limiting you to about 46T which, IMO, is way undergeared for a 20"-wheeled bike.
My bike came with similar FSA front rings so I switched to a road-double 52/40 to give me higher gearing.
It's difficult to make suggestions if we don't know what problem you're trying to correct. What's the priority? Go faster on road? Stooopid light components? (they won't last as long and they'll likely set you back thousands of dollars for a few ounces...)
A pair of high-pressure slicks makes a huge difference on a 20" folder for road speed. Tioga Comp Pools and Primo Comets are winners and Schwalbe Marathon Slicks are also good.
Take a look at my page for a list of chronological upgrades - many made just by finding bikes at the curb on garbage night - including the road-double crankset!