I think there are two different questions at hand here.
Will the same TRT dose maintain your same levels even after several blasts? The answer to this is yes, most likely you won't need to add more test to reach 600.
The second question is will you "feel" the same and that's where things get blurry. As far as results and how you feel, you will build a "tolerance". You will need more gear to achieve results while blasting and you may start to feel less satisfied with your 600 test levels.
Blasting on TRT can be a slippery slope. I think there is more risk than there is with those who cycle and PCT. When you are TRT you are injecting every week anyway, you arent going though the hassle of PCT and you aren't concerned with recovery.
This makes it easy to blast longer or more frequently than you should. And it can be tempting to get more into a cruise at 200-250mg rather than go back to a legitimate TRT dose.
Just my .02