I did some tests yesterday. I first did a test of the handbrake to see whether it would produce any braking even when the handbrake light is not indicating on the HUD. That test was positive.
After that I did an acceleration test with the handbrake slightly engaged. I then turned around to the same spot and did the same test without handbrake.
The first acceleration test was with traction control set to 1, found no difference in longitudinal acceleration and speed, see the graph below:
View attachment 1546935
The second acceleration test was with traction control set to 0. This time there was a difference, the run with the handbrake slightly engaged was a little bit faster. Note that I probably engaged the handbrake a little too much here, as it registered on the graph, see below.
View attachment 1546944
It's possible that the difference in the second test is due to wheelspin, with the handbrake slightly engaged it reduces the net torque at the powered wheels and reduces the amount of wheelspin. The fact that there was no difference in the first test could be because traction control kicked in to limit the wheelspin anyway.