We have both in our rental fleet, there is definitely a difference but I'd say it's mostly in build quality. The distortion (some barreling) and breathing (hardly any) are identical between the 16-42 DP and the 15-40, but chromatic aberration is slightly worse in the DP. I don't have our 30-80 DP here at the moment to throw up on my projector but from memory it's the same sort of difference to a similar range Optimo like the 28-76. When we asked an Angenieux rep about it he said they built the DPs to a lower resolution spec but I can't really see that on projection, except maybe very slightly in the corners at full aperture. The tolerance of focus drift through the zoom range might be a little less stringent. But past about T4 or 5.6 I don't think you could pick them to be honest.
The DP build feels different though, the zoom mechanics are lighter to the point where after some use and wear you might get a bit of creep if the zoom is tilted without a Preston fitted for example, and they make more noise. I would predict their lifespan to be shorter than an Optimo's. Some of these issues are of more concern to a rental house than to a production. If the budget isn't there, I don't think renting DPs instead of Optimos will be any sort of problem.
Most cinema release features tend to choose Optimos though, unless it's a 3D production where matched DPs often get used.
As is often the case with optics, it's that last 5 or 10% improvement that doubles the cost. Allowing the rear element to be much closer to the film plane reduced some of the optical design difficulties and hence the final cost of DPs also, but it's interesting that Angenieux have gone back to normal clearance for the new Optimo Style series.