When I was there late in February, the construction was right at the junction of 140/120, so it stands to reason the delay would be the same. I guess there's also the slide detour on 140 which can add a few minutes, but that one's nothing compared to the wait at the valley.
Since it had snowed a lot and I couldn't stay at the campground, I thought I'd give Mariposa some business so stayed and ate there, heading into the valley in the mornings. I got nearly the full half hour delay every time but once, so they're not kidding.
So unless they're working a different spot, it won't matter if you take 120 or 140.
Gary
Yosemite Photo Galleries: http://www.pbase.com/roberthouse/yo