GIMPACT 0.1 doesn't have margins.
About my machine is a Pentium4, 2.4ghz, 32bits, 1 Core.
And I've tested your GimpactDemo with GIMPACT 0.1, and you're right. It runs faster. The new GIMPACT 0.2 takes 2 times longer in my machine, and I don't know why ..

GIMPACT 0.2 uses the same algorithms than 0.1, but GIMPACT 0.2 is more dependant of Bullet.
I'm start to thinking that it could be the contact generation process. GIMPACT 0.1 uses its own contact filter process for giving the minimal amount of points. GIMPACT 0.2 uses the btManifoldResult class for filtering points.
However, I'm not sure if that is causing the slowdown. Let me doing some tests ok?