Here's why I wouldn't do this.
First of all a disclaimer: I own a P3D-, and I think it's a great car and would buy it again in a heartbeat.
In your case, however, you already have a great car, too. The RWD M3's extra efficiency and lower cost arguably give it a better price performance.
Furthermore, Tesla's weird strategy for performance cars is likely to get revised over time. What do I mean? The ridiculous notion that the big heavy monstrosity of a Model S has to be faster than the Model 3.
Anyone who knows motorsports knows that weight is the enemy of performance. The idea that the Model 3 performance has to be slower is one that makes sense only from a perspective of managing short-term cash flow.
I fully expect that as more reviews come out noting how much of a compromise the Model 3 performance is relative to sportier ICE equivalents (and I fully expect this to happen once the novelty of the electric drivetrain wears off), Tesla will up its game and come up with the *real* performance model 3 - you know the light weight, aggressive, sport-oriented performance, that so far outperforms similar ICE cars that any compromise is long forgotten. One that is not held back because of fears of stealing sales from the Model S line.
Sure, this may take a couple of years, but you'd still have an absolutely awesome car in the meantime, and don't have to throw away a bunch of sunk cost for a more marginal upgrade today.
