It's of course possible to limit speeds based on account profile/configuration, but not advantageous for the carrier or Tesla in this application. Both parties want you using the network more quickly rather than slowly. Typically, the only motivator is price, and I'm this case, Tesla doesn't charge for the service (yet). AT&T want you using their network less (faster speeds mean you're tying up fewer resources and/or making less noise for everyone else using the same spectrum in your cell).
Regardless, the above is moot. We already know the hardware they are using in the car: Sierra Wireless AR855x.
ATT Developer Program
It's HSPA+ to a maximum downlink throughput of 14.4 Mbps.