Actually, Car and Driver magazine tested the 2016 Volt on the track as 0-30 in 2.6 seconds versus 0-30 in 2.3 seconds for the S60. After than the Tesla S60 dominates more clear with 0-60 in 5.5 versus 7.8 for the Volt. MotorTrend found the Volt with a half-full battery in Hold Mode (hybrid engine manually driver-enabled) was 0-30 in 2.3 seconds and their original S85 review gave it 0-30 in 2.2. They found the Volt did 0-60 in 7.1. According to GM, the 2016 Volt's EV acceleration should be slightly better than its hybrid enabled acceleration. So, the Volt's 0-30 acceleration is actually a surprisingly close match to the S60 and even the S85 and is notably better than other non-Tesla plugins. GM gets the EV mode numbers by combining together both of its electric motors, which each have slightly different torque characteristics and gearing ratios.
You are referring to Mountain Mode. This allows the driver to optionally retain or rebuilt a higher than normal minimum state of charge in the battery pack to assist the gas engine in driving through mountain passes at high speed. It was more necessary for the first generation Volt which was heavier and had a somewhat smaller engine. As usual, the gas engine does not come on until the battery reaches a preset minimum charge level -- Mountain Mode raises this minimum set point from around 17% to around 40% (I'm not sure what the specific values are in the 2016 so I'm giving a rough example). This allows the car to dig deeper into the battery to help the gas engine, when needed.
Look on the GM-Volt.com forum for clues about how to buy it for a good price. There are a couple of high-volume dealers in the Los Angeles area with discounted prices and other deals are available such as so-called "private offers" from GM and sales arranged through Costco's car buying service. Some people have reported paying several thousand under MSRP. The LA dealers regularly ship to customers out of state and you might at least use an offer from one of them as a bargaining tactic with your local dealer.