For 20 minute finals you will prob need at least 5800mAh. Seems like those prices from the states are a bit high, and you may get stung for import duties. I have seen mixed reports about the MaxAmps ones as well with Gens Ace coming out on top of group tests.
Gens Ace do have a warehouse/web shop in Germany that ships all over Europe, you can find a 4S pack here;
http://www.gensace.com/epages/genspo...2P-HardCase-14
I've never heard of anyone using the Gens Ace own web shop so can't comment on how good the service is but it's worth looking at.