The IS-2's gun was easily on a par with the 88mm in terms of accuracy.
Here's an article detailing Soviet tests of the D-25T, you may question the source all you like, so long as you yourself have a source that disputes it.
http://tankarchives.blogspot.ca/2013/02/accuracy.html
Archive Awareness has a whole bunch of articles on various aspects on the IS-2 and related to dispelling the piling up myths that are rampant in WW2 online communities...
Here's another article detailing the effects of shells, be they HE or AP, on a Tiger II...
"Shot #1. Target: upper front plate. Shell: 122 mm HE-fragmentation.
Result: spalling across an area 300 mm by 300 mm. The welding seam between the upper front plate and the machine gun port burst on 3/4 of its circumference. Internal bolts holding the machine gun ball were torn off. The welding seam between the upper front plate and the right side burst, and the right side was displaced by 5 mm. The tank caught fire internally."
Here are those German welding seams, causing trouble again. If an enemy shell literally tears your tank apart without even penetrating it, that's bad news for you. The spalling and machine gun bolts flying around mean that the driver and radio operator would be dead..."
That's not good, especially when its the first shot with HE...
http://tankarchives.blogspot.com.au/2013/03/is-2-vs-german-big-cats.html
Finally, the fellows quoting German sources here saying how they've just destroyed an entire Soviet Tank army for no losses. Think about how many grains of salt you'd take when reading this combat report:
"Senior Lieutenant Makogonov's tank company destroyed and burned 17 Tiger tanks with direct hits. Of those, 8 were destroyed by Junior Lieutenant Pankov, 5 by Junior Lieutenant Pchelin, and 4 by platoon commander, Lieutenant Romashin"
Not saying whether its true or not, I'd want to find a German report to corroborate it, but it remains that its an interesting double standard you can have where you have every German report being taken as gospel, whilst every Soviet report is dismissed as propaganda or lies to impress higher ups.
Indeed, it's probably reasonable to assume that it was a lot easier for the Soviets to verify kills as they were advancing and so were able to study the battles which had been fought a lot better than the Germans who generally left those behind rather quickly...
Once again, not saying the report is true, but its about as reasonable as German reports out there as well...