I have recent experience with a 6.5 turbo..... sunk thousands into it. new IP, new hi pop injectors, replaced the injector lines, new non waste gated turbo, 3" cross over and down pipe with a 4" exhaust. Heath GL4 chip (the only advantage with a DS4 is the ability to command more fuel and bigger injector lines) not to mention over $1100 in parts to freshen up the IFS front end, or the 4 hours it takes to change the starter because you can't get to it.
Driving home one day one of my Mylex tipped marine injectors failed open, flooded the cylinder with diesel fuel and hydrolocked the engine. Pulled the engine to disassemble and inspect for damage. one head was cracked (on the side opposite from the bad injector) and evidence of over heating (if the temp gauge on a 6.5 reads 260 then its too late, the heads are trashed). found a crack in the #8 cylinder wall (also not the failed injector) right below the fire deck. and a crack on the middle main web from the register to the cam journal. Ok so this engine was toast (200,160 miles). Did some research and found out that I had a 599 block (this is supposed to be the strongest GM produced block) I bought a 6.2L to do a quick rebuild on and drop in. The block was cracked on the same main web in the same way. I bought a military take out 6.2, it was also a 599 block and cracked in the same spot. long story short .... I looked at 6 blocks and they were all cracked.
I got most of my money for the engines back by selling the internals and heads, and recycling the blocks. I ended up taking off all my performance parts and scraping the truck (due to a hole in the frame)
for a 6.5. If you think the heads are cracked, they are.
if you think the head gaskets are blown, they are.
if you think an injector has gone bad, it has.
if you have not mounted the PMD on a heatsink and relocated it to the front bumper, you better.
Most DS4 problems can be fixed by cleaning the grounds.
If the engine has ever been over heated then the heads are junk.
a used oil analysis is very important on this engine.
other than that, its a good engine, mine was at 260 rwhp and 503 ft/lbs torque just buy doing injectors, a turbo, exhaust, and a custom tune. It would pull hard all the way to the point the computer defueled the engine which was fast enough to bury the speedometer and put down 15 psi boost. It could also smoke it's tires in 1st, 2nd, and 3rd with a 4L80e and 4.10 on stock size tires.
if i ever bought another I would buy a drop in P400 optimzer or build and engine from scratch with a new GEP block and heads.