We actually have that date available via the Steam achievements data. If you complete the game with hardcoe mode on, you get the hardcoe achievement, as well as an achievement based upon which quest line you used to finish the game. If we assume that everyone who played through the game only ever got one of those 'finish the game' achievements, then we can figure out the minimum percentage of players who played with hardcoe (assuming the percentage of people who finish the game in hardcoe mode is equal to the number of players who play the game in hardcoe mode).
The achievement percentages look like this:
All or Nothing: 4.7%
Veni Vidi Vici: 3.2%
No Gods No Masters: 10.8%
Eureka!: 6.5%
hardcoe: 4.7%
That's a total of 25.2% of people actually beating the game (in this assumption - in reality it's even less than that), with 18.6% of them playing it in hardcoe from start to finish.
Remember that this understates the number of people who played in hardcoe mode.
We can put an upper bound on the percentage of players who played in hardcoe mode by assumimg that the everyone who played in hardcoe mode got the No Gods, No Masters achievement. That gets 43.5%.
The real answer is going to be somewhere between those extremes - somewhere between 18.6% and 43.5%.
But it's nowhere near 'one or two percent'.