In any system, performance is limited by the weakest link. In the Tesla drive train, I see 4 possible bottlenecks:
1) The motor's rating
2) The inverter's capacity
3) The current and voltage limits of the battery pack
4) Software limits which artificially limit the performance
Tesla has been up front from the start that software limits the top speed of the car. They have never said anything about software limits on performance, but while that's possible, I don't see where they have much need to limit performance on the small battery cars. For people who are trying to decide between a 75 and a 90 the slower acceleration of the 75 is not going to be a significant factor for most buyers. (Though there are always exceptions.) The difference in range is enough to distinguish between the two versions.
Going back to the 1st generation Model Ss, they all had the same hardware with two different batteries and changes in software. This article is written by someone who upgraded an original 60 to an 85. Tesla swapped the battery pack and changed some switches in the firmware to bring the horsepower up to that of the 85.
Life With Tesla Model S: Battery Upgrade From 60 kWh To 85 kWh
Tesla did limit the output of the motor in firmware, but only because the 60 pack couldn't deliver enough current to safely support the higher performance.
As far as we know, the 75D and 90D had the same hardware with different battery packs. The 0-60 performance difference between the old 75D and 90D was about the same difference as seen in the original 60 and the 85. The software limited 60/60D had the same 0-60 performance as the 75/75D, which indicates that whatever the limiting factor was, it was the same between the software limited and unlimited cars.
To me, it's always made a lot of sense the lower performance of the small pack cars was due to the lower power output from the pack and any software limits were just for safety and nothing else.
The more cells you have, the more power is available when accelerating hard. The original 60 to 75 had some performance improvement because there are more cells with higher capacity in the 75 pack. The 85D to 90D did not see any reported improvement which may indicate the bottleneck was somewhere else once you got into the larger packs, or it may just be Tesla didn't bother to update the specs. The 100D has more cells than the 85D and 90D did, and it got a small performance boost.
It's quite possible that Tesla is now shipping cars with new inverters and new motors, but I can't understand how doing this would improve the 75 that dramatically unless there was some other change that allowed more current from the battery pack.
I've been predicting that the small pack car was going to get updated to use the same modules as the 100 pack cars sometime soon. They are also doing everything they can to sell off the old 90Ds as fast as possible. The large pack has 16 modules, if you make a pack of 14 modules with the same modules, you get an 89 KWH pack and a tad more cells than the old 90D pack.
If the updated 75 is really a software limited 89 KWH pack, the numbers line up perfectly. The new 75D has almost identical 0-60 performance as the old 90D and if it has 14X modules from the 100 KWH pack, it would make perfect sense because it's cell count is very close to the old 90D pack.
It would also make sense they would be quiet about the new pack until all the old 90Ds in inventory were sold off. It would be confusing to customers which technology tree the pack was from. The slight improvement in 100D acceleration can easily be explained by Tesla just getting around to updating the specs for the pack that now has more cells than the old 90 pack and can deliver about 15% more current.
I could be wrong about all this. Tesla is notoriously hard to predict.
It does make sense they are updating the inverter and motors. The ones developed for the Model 3 are a generation newer tech and the old Model S equipment. Whereas they will likely contribute to higher reliability down the road, I don't see them contributing dramatically to performance alone because that wasn't the bottleneck, especially with the 75 KWH cars.
When the updated 75s start hitting the showrooms and into customer's hands, I'd be interested to see any pictures of the battery pack's label.