Eh... There was a toll lane added from just outside Boulder part way to Denver. This has not generated more traffic, but it has generally speed up the road (sometimes to an undesirable extent as faster moving vehicles make more noise).
They didn't change the two lane in each direction structure, they just added another lane on each side which can only be entered/exited around exits.
A couple things I've noticed, most mid-day traffic occurs in clumps. You can usually take one segment of toll road for $0.35 and exit back into a mostly empty highway. Also, the toll lane people usually move above the speed limit. This speeds up traffic in general because people seem to tend to move into the right lane when someone blasts past them. A few people do it and most slow drivers will get the idea. And it also discourages lane switching which is a significant contributor to traffic. Once you're in the toll lane, you're committed to it until the next exit.
The problem with just arbitrarily adding more lanes is than lane switching is a major cause of traffic. Every time someone switches a lane because their lane is too slow, they usually cause the car behind them to have to slow down which causes a ripple effect. Add this to complexity of an 8 lane freeway and you may have created a nightmare for travelers. Granted in over populated areas like much of CA, congestion is somewhat unavoidable. (I grew up in the LA area)