I have the same conundrum...
2002 - 911 Carrera (coupe) fairly modded for auto-x - corner balanced and all that.
2015 85D fully loaded (except the 3rd row seats)
I have had the 911 since 2010 and have tracked it a lot and auto-x when I have time...I love the car. That said, since buying the Tesla I have driven the 911 maybe 10x...I even tried to take the 85D to an auto-x day. I did ok with it but the 911 is another animal...it doesn't have the instant torque but the Tesla cannot come close to the lateral G's that the 911 pulls.
My bottom line is: keep the 911 until I can upgrade to a GT3 someday...if I let it go and upgrade to a P90D I will miss sideways G's (a lot). The MS just will not corner like a 911 - ever.
If you don't track your 911 (or auto-X) you will likely be able to never look back, but if you enjoy pushing your 911 to its limits - you might consider keeping it. One thing tho, I used to commute in the 911 before I got the Tesla, but not anymore...day-to-day driving in Northern Cali traffic is a Tesla job, not a 911 job (at least that's what my 911 clutch says
).
Tough decision but also a good problem to have