So first off, obviously it costs more. What do you get for that higher cost? Higher top speed- to me that's irrelevant, no one is going above 100 MPH in a normal day. A spoiler- eh ok cool. Aluminum pedals- eh ok cool also. The acceleration- all teslas go really really fast and you can buy perfomance boost if you really need more acceleration. You are also giving up range to do this which to me is more important.What makes you say that getting a M3P will make you sink more money into it?
Now the downsides:
1) The tires- as I have said before they are performance tires not all weather. They are not only useless but dangerous in temps below 40F and/or snow. They also really need to be stored in a place that doesn't freeze as well. So you need to buy a set of tires for winter thats 2-4k AND you need a place to store the performance ones. Plus time and labor to change the tires. Also using non performance tires can impact your acceleration and range even further negating some of the previously mentioned benefits of the P.
2) The brakes- the large performance brakes require more maintenance and are more prone to rust if not used/car is stored. Also these larger brakes limit your options for replacement wheels/tires
3) The 20 inch wheels themselves- require a larger tire that is more prone to damage on pot holes and uneven roads. Also may be a more bumpy ride
4) Insurance- As this is a performance car some insurances may charge a higher premium (on an already high premium) to cover the car
As such the benefits of the car, to me, don't seem to outweigh the drawbacks. However, if you live in an area with excellent roads that never gets cold and you plan on taking the car to the track, then yeah this might be the car for you. But if you want it as a daily driver you are likely going to have a lot more issues then someone who got the LR or SR+. Ultimately its up to the buyer what they want but I strongly suggest people looking to go from LR to P strongly do further research.