My son has has a Bluespark Automotive box on his BMW 435D and its made a huge difference. Either more go or better economy - you choose.
They also do a 30 day trial so if you don't think it works you can return for a full refund. Whichever box you choose, its best to get one that has two channels and so controls fuel and air. Cheap boxes just pump more diesel into the engine resulting in clouds of black smoke from the exhaust and problems with the egr.
The Bluespark box has a horrible reputation on Briskoda for poor customer service and for blowing up Fabia 2 vRS Twincharger engines. Now, it could be argued that that particular engine is made of chocolate anyway (something like 35% have been replaced under warranty for excessive oil consumption) but Bluespark just flat out refuse to accept any liability and continue to sell a tuning box they have to know is dodgy in that application.
Your point about the number of channels is valid, the single channel boxes match only the mass air flow sensor, so they add more fuel as the ECU calls for more air. The adjustable ones simply let you adjust the amount of extra fuel being added. That's linear with air requirement and when the amount of fuel exceeds the amount of air required, it generates soot in the exhaust. These particles of unburnt diesel are removed in modern cars by the diesel particulate filter. When the soot is excessive, it clogs the diesel particulate filter and the car stops.
Two channel systems also monitor boost from the turbo so they can effectively force more air AND fuel into the cylinders so they don't tend to suffer from the DPF clogging issue however on a twin turbo engine, like the SLK CDi250 they don't know which turbo is in use so they are either set to pull hard on the primary turbo and under-boost the secondary or they are underboosting the primary turbo and come in like a sledgehammer when the secondary boosts up. Neither is very satisfactory because one tails off and the other feels like epic turbo lag.
So, now there are three channel systems which monitor the MAF, the turbo boost sensor and the camshaft sensor, so it now also knows the engine revs. When it knows the engine revs, you essentially can map the air and boost to match the original responses of the car. So when the revs are low, you add less air and fuel because the turbo is smaller, as the revs and boost rises, you can add the correct amount for the bigger turbo.
This is what a proper remap does, and because it's written to the ECU directly it doesn't need the box. Three channel systems are MUCH better than two channel systems. Only DTUK has a three channel system. DTUK are resellers for the German manufacturer, DTE, so all the DTUK tuning boxes are TUeV approved and have rolling road printouts to verify their minimum increase claims.
By far the best add-on box is the D4 from Brabus which is a true second ECU that sits on the CANBus and reads all the signals from the car and ECU. This is one reason it is so expensive. It's sufficiently clever that it knows when you are in ECO mode and just gives you the extra 100Nm torque but if you turn off ECO you get an extra 35PS as well. It can only do that because it's reading the CANBus. If you can afford it, Brabus is the box to buy.
I do apologise for the epic length of this post, but sometimes you just need to explain stuff!