Can AC/DC conversion losses from solar panels be avoided with a Tesla Powerwall?
Inverter efficiencies are typically horrible.  If you run the inverter at maximum power output, the efficiency is typically poor (95%) but of course you almost never run at full power, so the efficiency is horrible (75%), and sometimes you only take a trickle of power, so the efficiency is 0 to 50%.

If the system is correctly sized and designed for efficiency, then 95% transfer efficiency is achievable. 

Any inversion has similar losses. DC/AC then AC/DC loses efficiency, but it's only twice as bad as a single DC/DC converter, so in a well designed matched system you're only losing an extra 5%. 

The efficiency you get will depend on the design of the systems you use. Typical solar inverter systems are designed to maximize the utility to the home owner, that is how useful the system is. If you are willing to accept a system that is designed with only one thought in mind -- using the minimum number of solar panels -- then you can certainly buy a system that is more efficient. (The systems I work on are better than 1%)

Now that panel prices have dropped, it is almost always cheaper to buy more panels.

Having said that, if your purpose is to charge a car, not sell power, then it is reasonable to look around and see what kind of systems you can buy. A correctly matched DC/DC system will be more efficient than a standard (one size fits all) solar inverter system.