For Japanese I think the best experience is Nikkai Associacion Japones -- look it up on Guia Oleo. A very nice selection of food, very well done. Also if you google Nikkai you may come across the Japanese Cultural Associacion's newsletter, in which they list a lot of different Japanese and other Asian restaurants, if you can't find it let me kow and I'll have a look through my bookmarks and see if I've got it somewhere.
Garden is supposedly where the Embassy people go... I was there for Chinese New Year a couple of years ago, and the food I think was very good (... clarify, good for here, I'm from Vancouver so I will probably never be happy with what I find for Asian food here), but the interior was more English pub...
Garden - Cocina China | Guía Oleo - Restaurantes de Buenos Aires
Lotus Neo Thai is good food, not really authentic Thai but I felt it was miles above Empire...
I think BuddhaBA is supposed to be decent, and there's some others listed in Guia Oleo. Do as recommended, scout the restaurants with the most Asians. Also if you go to Barrio Chino on the weekends you can get decent street food (dumplings, pork buns, etc)
The thing that you have to remember is that even in a place like Vancouver, Chinese food was pretty much the Americanized version until everyone moved over in the 80s coming up to the HK handover. It was only with the flood of immigrants that the restaurants started to feel they could move beyond Chop Suey and General Tso's Chicken. Buenos Aires is in the Chow Mein Chop Suey Sweet n Sour Pork phase... and there's not a lot of signs that that's going to change. We are talking about a country that only accepted sushi once it was filled up with cream cheese, lol.
If you stay here long enough you'll become like me -- every flight home means a desperate run for dim sum right after the plane trip, shower can come later, hahaha.