I believe you that its technically possible to make 1000mg/ml, but I also don't believe that 99 out of 100 people would be able to take it without experiencing epic side effects. I guess you could technically buy it to dilute it with other oils or other anabolics in your cycle just because it might be more test for your buck. But I refuse to believe that it was "great" for anybody but a tiny minority of people.
It's more likely that this supposed "1000 test" was actually a far lower concentration.
I experienced terrible flu like symptoms and massive PIP from 400/ml. My endocrinologist claims that the closer test gets to its saturation point, the side effects increase exponentially as it become more difficult to absorb. He said that at higher concentrations of test, your body will attack it with an immuno-response similar to an abscess or virus, hence the extreme PIP inflammation and flu-like symptoms.
I was ultimately only able to tolerate the 400/ml after diluting it with sterile cottonseed oil. I think the plausibility of anyone injecting test 2.5 times more powerful without side effects is almost zero. Shortly after this, the UGL I dealt with stopped making the T400 because it received so many complaints.