A number of years ago I had a 2008 Ford Mondeo 1.8TDCi. It was modified somewhat and used to give off a very slight smoke haze under load.
I worked close to home for 6 months, less than 5 miles and in that time I had no real opportunity to open her up and get it properly hot. The VNT turbo ended up sticking because of the soot build up. If I put my foot down I would get an overboost DTC and the car would go into limp mode.
I ran two cans of the BG 244 through the car over 2 tanks of fuel. After the 2nd tank I put my foot down up a long, hard incline. I couldn't see out of my rear view mirror, there was thick, black smoke pouring out of the exhaust through 3rd, 4th and it cleared up through 5th.
After that the engine run better than the day I bought it (although, it did have 87k on the clock and it was 3 years old when purchased). Despite using premium diesel (usually BP Ultimate of Shell V-Power) I started putting a can in with a full tank every 10k or so. Always a slight noticeable improvement with every can.
I think additives have their place and continue to use BG244 in my Volvo at regular intervals.
After my Mondeo I bought a brand new 2015 Focus 1.5TDCi. I used a can of BG 244 every 10k but never noticed much of an improvement. That said it was kept completely stock, had only premium diesels from day one and throughout it's 30 month/66,000mile life it led with me it spent most of it's life doing trips longer than 20miles and usually of speeds above 60mph.