I dont know if my input is going to be relevant or not, but thought i would throw my 2 cents in.

My bro just bought at 118TSI DSG, and I have a Polo GTI with very LIGHT mods.
The 118 feels as fast as the GTI, but smoother... the GTI comes on a big stronger when the turbo kicks in.

.... I dont think you would be happy going to the 90 from the Polo GTI as far as performance goes... and I dont think the 10-15kW you get from the flash would be enough.