We already know it will be significantly better in at least one way - the Maximum Plaid acceleration will probably give it a ~2.5s acceleration from 0-62 miles. And Porsche's increase in mileage 5 years from now compared to current top of the line Model S seems tiny as well, and my guess is the new top of the line Roadster will have at least 20-30% better mileage as well.
[1] http://www.theverge.com/2015/7/17/8994649/new-tesla-roadster...