Ah yes, that pesky science thing keeps popping it's head up in discussions amongst the bro's. In the current anti-intellectual climate it's a bit like a game of Whack-a-Mole, some of us keep reading and posting the studies, only to be shot down by the bro's who say "I don't care what the science says; I know that (some ridiculous idea) is true!"
To be honest, I am always surprised to see questions like this debated at all, because Shalender Bhasin and others settled this question rather definitely with his groundbreaking series of studies back in the late 1990's and early 2000's, commonly referred to as the "600mg" studies, which are often cited and just as often misinterpreted by those who wish to read into them what they will. I've spoken to Dr Bhasin on many occasions (he is my endocrinologist), and he had two purposes in doing the studies: firstly, to show that test and other steroids work, because of course this was back in the day when bottles of steroids still came labeled with the warning that they provided no athletic benefit. And secondly and more importantly, to show that the results of testosterone and other steroids are dose-dependent.
"This study demonstrates that an increase in circulating testosterone concentrations results in dose-dependent increases in fat-free mass, muscle size, strength, and power. The relationships between circulating testosterone concentrations and changes in fat free mass and muscle size conform to a single log-linear dose-response curve. Our data do not support the notion of two separate dose-response curves reflecting two independent mechanisms of testosterone action on the muscle. Forbes et al. (22) predicted 25 years ago that the muscle mass accretion during androgen administration is related to the cumulative androgen dose, the product of daily dose and treatment duration. Our data are consistent with Forbes’s hypothesis of a linear relationship between testosterone dose and lean mass accretion; however, we do not know whether increasing the treatment duration would lead to further gains in muscle mass."
The guys using low or TRT doses of testosterone got lesser results than the guys using 300 or 600mg per week, and the higher the dose, the better the results. You would think this would be common sense, but it was not yet accepted back in those days.
For what it's worth, Dr. Bhasin is very against using any more than TRT doses of testosterone, and believes that the lower the dose, the better for health. He disagrees with the use of any other steroid other than testosterone at TRT levels. And he knows that he could never replicate these studies today, because it would be considered unethical to give such high doses of testosterone to the test subjects.
To answer the original poster's question "How good can you look on test only?", the answer is, that all depends on the amount of test you are using, and how hard you are working. You can maintain a physique you built with high doses on TRT levels, at least for a while. And of course TRT doses are always going to be better than nothing at all.
All other things being equal (which they never are: training, nutrition, recuperation etc), you are always going to get better results with a higher dose than a lower dose. But you can certainly look good on TRT levels of test only, depending on what your definition of "looking good" may be. Are you going to look like a pro bodybuilder in contest shape? Of course not. But you are certainly going to look better than the average guy on the street, as long as your training and nutrition and recuperation are on point, and of course depending on your own individual genetics. And there are plenty of guys out there, only using TRT doses, who look great, and many others, using TRT doses or higher, who don't look so great.
So as with most things in life, the answer to the original poster's question "How good can you look on test only?" is: "It depends." It depends on the dose, and it depends on you getting all the other pieces of the puzzle in place.