According to the design schematics on the tollway website, the extension is only 2 lanes in each direction through the 290 interchange; a lane is dropped westbound after the 290 exits but is added back downstream, and eastbound the 3rd lane is dropped for 290 southbound but added back into the mainline from the second yet-to-be constructed flyover. The west bypass (I-490) has always been planned with 2 lanes in each direction, but will also be constructed with a paved 30-foot 'shoulder' along the eastern side of the travel lanes, allegedly for future mass transit.
Reducing lanes for initial construction isn't a bad idea. As a previous poster noted, until I-490 (2020s) or the western terminal (2030s) are open there won't be hoards of traffic. Like the western section, design accommodations will be made to allow future growth.
BTW I haven't heard an explanation for the exits/mileposts starting at 6. Is the eastern terminus of the US20/Elgin Bypass 6 miles to the west?