There is not an awful lot of difference between the 20G and a 3071. The 71 has a 3mm bigger compressor wheel. My gut feeling is that with a 2.1 litre that you would get better response by going for a TD05-20G or a GT2871.

Cobb has had great results on a TD05-20G on the 257. Especially once you run e85 and higher pressure ratios.