I specifically mean the current-generation vehicles with current-generation sensor suite, regardless of how many times they update the computing power. So "HW3" is the compute board upgrade but not a sensor upgrade. I don't believe it will enable L4/L5 driverless robotaxi-level autonomy, at least not before the cars are hopelessly outdated anyway. (So, something like 7 years maybe? By which time the whole landscape will have changed and it won't matter -- and Tesla won't be updating AP2/2.5 anymore.)
If AP3 (as opposed to HW3) comes around with a better sensor suite and more redundancy, then all bets are off. But current-generation vehicles will never get a retrofitted AP3 sensor suite. I would not be surprised to see something like "AP2.9" which is HW3 computer plus some minor enhancements to the sensors and wiring to allow better redundancy, maybe heated radar. AP2/2.5 cars will not get this (hypothetical) AP2.9 upgrade because it's too expensive to change wiring and sensor mounting, if the mounting needs to change. This may give you better performance but I still don't think you're in the robotaxi zone.
Still hypothetically, AP3 would be something like 360-degree radar coverage and/or short-range (small) stereo camera pairs providing 360-degree coverage with sensor cleaning, plus a true long-range forward stereo camera pair or lidar, plus more redundancy in the electronic braking, steering, and power delivery systems, plus ~5 HW3 chips to crunch all that data. I could get on board with something like that providing robotaxi-level autonomy, circa 2024.
Obviously no current-generation vehicles are getting that retrofit.