The OP is back. Been collecting sample prints from 3 labs. Staring, comparing, resting, repeating. Even brought in a couple of "kids" with young eyes to get further analysis.
My conclusion is that lab prints are OK for me, and I've confirmed the retirement of my Epson 3880. Like many things in life, I would describe this exercise as a good example of the "80/20" rule, but actually closer to "90/10".
I can get a 10% overall improvement in image quality with the Epson 3880 over labs, but at a 90% increase in effort and cost. OK, maybe only a 50% increase in effort/cost. But even that lower ratio is beyond my personal threshold of pain.
Two mitigating factors. First, I just don't print enough to amortize the cost of a printer and the effort to maintain ink and paper stock. Second, the audiences for my prints are simply not able to see or appreciate the 10% advantage of inkjet prints.
My test prints included some color challenges. Large areas out of gamut. Both out of sRGB gamut and certainly the printer gamut. But the lab printer conversion engines did a fine job.
My test labs were Mpix, ProDPI, and my local Costco. All prints on Luster paper, no lab color corrections. The loser in all comparisons was Mpix. The Mpix prints were all darker, maybe 1/2 to 1 full stop darker. Shadows blocked up much more than the other labs. Plus the Mpix prints looked more saturated (over saturated). The Mpix prints were $7.50 plus high shipping cost.
ProDPI and Costco were a virtual dead heat. I can see slight differences, but only in side-by-side under very close inspection. ProDPI and Costco prices are the same, about $4.50 for 11x14 luster. ProDPI has free shipping, took 5 business days. I drive 5 miles to my local Costco. There are 3 "pro" labs in town. Didn't use them. They charge $14 to $17 for an 11x14 luster print. Same paper, same machines as Costco/ProDPI.
I chatted up the lab manager at Costco, and was quickly convinced she knows her stuff and takes pride in the work. She seemed happy to talk to a customer who has some knowledge and interest in their process. She knows colorspaces (they accept and convert Adobe98 and Prophoto, even accept tif). She described their maintenance and calibration at length. She was excited because in a couple months they will replace their Noritsu with a Fuji Frontier.
I'm only left with one mystery. I converted a Prophoto image to the Costco profile in three different steps. One was Prophoto direct to printer profile. One was Prophoto to Adobe98 to printer profile. One was Prophoto to Srgb to Printer profile. The Srgb version looked best. It was closer to the original. Both the Prophoto direct and the Adobe98 version showed darker tones in areas that were out of gamut.
So I sent 3 copies to Costco. One in Prophoto, one in Adobe98, and one in Srgb. The prints came out the same as the conversions. Srgb looked best, Prophoto and Adobe98 looked the same. That made me think Costco was not really handling the Prophoto and Adobe98 conversions correctly. But the differences were only in out-of-gamut areas. Other colors were the same in all three prints. Still scratching my head on that one.