I've called Dell about inferior replacements that showed up onsite, and Dell's policy was "they never advertised performance, only size and spin speed" and this part met those qualifications, and was the approved replacement part.
I've also seen many techs with the right part screw up rewiring the board, not being able to get USBs working again (or thinking they had a bad board when they simply failed to double check if the power connector was in the right pins). I've seen techs there to replace a hot spare kill a raid by pulling the wrong drive a dozen times, and I've seen techs who can't do anything beyond replacing the part (don't know how to even get in the BIOS to establish the new drive as a RAID member).
After market replacements are not covered under warranty, so I can't fault Dell for screwing that up. As far as replacing them with better at one time, then lesser (but still better than original) later, I've never seen that happen once. Per my understanding, Dell, as does HP and Acer, track each part by ID number in a build, not a default config of generic parts. They know exactly which part is in each machine, because almost everything they sell is custom order. I've even been told before that a part in my machine was not covered because it was not the part they had in their database, because a tech came out to replace 2 drives in 2 machines, and put the wrong one in each, swapping serial numbers.
Fact is, more often than not, and I've seen Dell contractors in 11 cities, the guys coming out are not even as good as BestBuy's in-store techs/geeks. They're generic, $12/hour part jockeys with little training and no knowledge who are on staff merely because that's all a company can afford to pay someone who gets $70 flat fee for an onsite job, and no one who knows more takes pay that low.
Real service? you find it at your local outlet or service center, not by rent-a-geeks. Apple's in store people ROCK. When IBM had shops around town, their people knew their stuff too. When I worked for a compaq certified server shop, we had to continually train people, even in high class products we didn't sell. I was in training 3-4 days a month just for compaq, another 2 for HP, Then DEC, NEC, Okidata, and more. If I wasn't inside a machine, I was inside a book, or a classroom, and failure to keep all our techs to that standard meant loss of our contract for repair. Then comes along some company faxing us offers to fix Dell stuff, flat rates, no materials, not even access to service manuals, at at lower pay and no travel expenses included. It was a joke, and we told all our customers that what you got when you bought Dell, a guy dispatched by a fax with no data...