So much guessing in here.
The modern Model S (including Plaid) uses a 110s battery configuration. This gives a nominal voltage of 401.5V
The Model 3 uses a 96s configuration for 350.4V.
Yeah, the Model S will have more top end. 15% more voltage means a lot even if you change nothing else. There's some non-linearities in here too which mean each volt is worth more than the previous one. Which is why Porsche and others are going to 800V.
As for carbon sleeves on the Plaid- this is because they wanted to flat rate the car to 1000 HP from 0-200 MPH. Their innovative laminate and magnet structure along with carbon winding allows them to reduce the air gap between the rotor and stator. This increases torque at the same voltage. This is not free- this requires more current at the same voltage, but it means you can actually get that current instead of being limited by back EMF. Notice though they needed to go with 3 motors to get that 1000HP. What they are really doing is under-rating each motor at low speeds, so that they can keep the power flat all the way up in software. But it pulls all that off beautifully. I'm sure they very specifically tuned the whole system carefully for this. The Plaid even runs lower gear ratios (14.6K RPM rotor speed @ 162 MPH vs 18,500 for a Model 3 or Model S @ 162 MPH) to trade current and voltage capabilities of the battery.
What do we have in a Model 3 today? Well, we know the sum of the rating of the front and rear motors is already more than the battery can handle. So the battery sets the max current limit for the vehicle. If Tesla didn't care about the battery, the car could already be faster 0-45 MPH. Watts is watts, so you can't fix that with a different motor. All these motors are already 98%+ efficient, so there's no magic there.
So you could put a carbon rotor in a Model 3, and then you'd be able to keep up the power at higher speeds. This would slightly shorten the 45-60 MPH time, but not enough to be a big deal for overall 0-60. And the reality is nobody really cares about 60-100MPH times except racers. Plus, Tesla wants the Plaid in the 2's and the Model 3 in the 3's.
If you really want better 0-60, you need more watts from 15 MPH to 45 MPH. And that's the battery that is limiting that, even today. The motors are not the limit.
Remember- max watts out of a battery is at first order set by how big the battery is. And the Model S has a huge advantage of having 20% more capacity than a Model 3. You can't just re-organize the cells in a model 3 to be higher voltage, because then your allowed peak current is lower. So even a Plaid rotor in a Model 3 would give up much earlier than in a Plaid- but there's a good chance it could be flat rated to 100 MPH.
Oh, then there's the fact that the front and rear motors in a Model 3 aren't even the same architecture and they can't put a Plaid rotor in the front, so that factors in to the overall power curve at high speeds too.