If Tesla embraces the new HET DC motor that is under development at Linear Labs (or I would imagine a proprietary version thereof), they won't need to improve battery technology much. Motor efficiency will drastically increase range (and performance specs) while possibly even reducing storage requirements. Once this motor becomes industry standard, I have no doubt we will see ranges between 400 and 500 without any change to battery. I give it two years before Elon engineers a similar motor, or Tesla buys patent rights to Linear Lab's version.
HET electric motor massively boosts power, torque and efficiency, reduces weight and complexity
Add to this the breakthroughs in battery tech, such as "solid state" or "dry" lithium ion packs under development by Fiskar, or even more promising, the new Li+ inorganic electrolyte battery being tested by Innolith as part of Hagerstown, PA "Gridbank" project.
Electric car battery with 600 miles of range? This startup claims to have done it
Put these advancements together, along with whatever brilliant engineering Elon can cram into it, and you may have an EV with over 1000 mile range, that outperforms modern P3D+ or P100D models in the next 5 years.
(I know Fiskar is a bad word in the realm of TESLA, but you can't discount the work they are doing to contribute to advancing battery tech)
We live in exciting times, I tell you.