I'm linking a thread where rate of fire was discussed specifically between the 75mm and 76mm Shermans, but along the way data was presented regarding ROF for the T-34s as well (later pages, 7-9). Bottom line is that ROF for the T-34s is no slower than published documents indicate. By that same token, the ROF for the Shermans is no faster than published documents indicate. Absent authoritative sources that contradict the data HTC is using, it is doubtful any of the rates will be changed.
The disparity in rate of fire is due in large part to the ergonomic conditions for the crew, which the linked thread will demonstrate.
http://bbs.hitechcreations.com/smf/index.php/topic,289361.0.html
IMO, when there is credible information that says "15-20 rounds per minute", I would hope that the heavier and longer round for the M4A3/76mm would account for a slightly slower reload time vs the shorter and lighter 75mm. The variable reload rate is given for a reason. Obviously, the experience of the crew is prime, then their training level, and then the actual mechanics of loading/unloading/reloading the gun. Since the LVT-4 can hurl rounds down range every 3.6 seconds from an open turret, I would like to think HTC would recognize the logical transgression of heavier and longer rounds in a confined space would add time between shots. Granted, the LVT-4's 75mm M2 howitzer and the M4A3/75mm (M3 L/40) fire the exact same HE round, but again the LVT-4 has an open turret which allows for greater freedom and ease in reloading.
Adding a few 10th's of a second to the M4/75mm Sherman could hardly be argued against when compared to the LVT-4, and then adding a few more 10th's to the M4/76mm Sherman could hardly be argued against when compared to the M4/75mm and the LVT-4.
Heck... what about HTC changing the reload rates every so often to the tanks so reflect the skill and experience of the tank crews??? The Panther G has a reload rate of "6-10 rounds a minute". Thankfully, in AH we are on the better end of that scale. In fact, I think all of the tanks are on the better end of the reloading time scale. No harm in mixing things up from time to time??? Would the AH world be turned upside down if the Panzer IV reloaded at the same rate as a M4/76 at 3.8 seconds??? Would a veteran crew in a T34/76mm cause havoc if they reloaded at 5 second??? Food for thought.