Mostly. Even though I'm using the TEST version of Authorize.net API, it only works if the "Live" server is selected in uStore. I didn't get a precise answer on why this is, but my guess is that the "Test" version in uStore is meant to work with the sandbox server for Authorize.net, but it is confusing because the live server has both a "Test" and a "Live" mode.
I also had to deselect the relay response setting, otherwise the transaction times out and fails.
Hope it helps!