Well, all the signs point in the right direction - the 'standard' MkV Golf (ie non-GTi) is apparently much more dynamically capable than the comparable MkIV, so a breathed upon GTi should be impressive.
I don't think that the initial launch car (200bhp 2l fi turbo lump) will be some sort of record breaking rocket sled, as VW are now inherently heavy cars.
As much as people say they'd like it, we're never going to hark back to the days of the MkI and II - modern cars now have to conform to a much higher crash standards, dictating stronger (and therefore heavier) cars. VW have built a reputation on quality, and their customer base demands modern 'luxuries' such as air con, loads of sound deadening, esp, abs, power steering etc etc. which all add to the weight.
If VW wanted to, they could create a MkV that goes and handles like a MkII, but in order to do so they would need to strip the thing bare!! And that wouldn't be economically viable, as modern cars without climate control, abs, traction, power steering etc etc don't sell to the mass markets, only the hardcore minority.