I have not found any data that will counter Sebastianus. It doesn't look like anybody else has either because the only counter arguments come down to it is impossible to compare cremating human corpses with cremating farm animals or more and more eyewitness testimony. Neither of these counter arguments holds any water.
The denier claims about cremations at camps like Treblinka have been addressed
here, specifically
The "Duration of Cremations" section notes that at Dresden, a single pyre on the grate used could burn 500 corpses, and it took five hours to burn them. The grates at Treblinka were 4-5 times the size of the Dresden grate, meaning that each grate could burn 2000 to 2500 corpses per day, with the same packing/density of corpse layers as that used in Dresden.
If there were two grates of that size, burning one pyre a day lasting five hours each would cremate 790,000 corpses in 158 days (just over five months).
Burning two pyres a day (that is, two 5-hour burns in a day) of 2500 bodies each could cremate 790,000 corpses in 79 days (a little under three months).
Burning bodies round the clock (four 5-hour burns of 2500 bodies each, with a four hour break), two grates of that size could cremate 790,000 corpses in just over a month.
A single Dresden-sized grate running round the clock (four 5-hour burn shifts of 500 corpses each, with four hours off, per day) could cremate 790,000 corpses in 395 days, or a year and one month.
Two Dresden-sized grates running around the clock like that could cremate 790,000 corpses in 198 days, or six and a half months.
In short, if the Germans at Dresden could cremate 7000 people in 12 days by using a single grate burning 500 corpses a day in a burn that lasted 5 hours, they could
easily have cremated 790,000 people in seven months at Treblinka.