This test was performed on a Sony NEX-F3 (borrowed) camera. This is a 16 MP APS-C sensor.
Here are the full res images:
Canon FD 24mm f/2.8 with Speed Booster, shot at f/8 (becomes 17mm at f/5.6), 1/2000 sec, ISO 200
Sony 18-55mm f/3.5-5.6 at 18mm, f/8, 1/400 sec, ISO 200
The camera metered differently for these two shots. The shot with the Sony kit lens was 0.8 stops too bright--I adjusted it down in Lightroom to match (more or less) the Canon FD image. The white balance was also different. You can also see (maybe) that the 24mm lens (which gets reduced to a focal length of 17.14mm) is a bit wider than the 18mm kit lens.
Canon FD and Speed Booster
Sony Kit lens
Here are two images inside, a bit closer to the camera (less than 1 meter):
Canon FD 24mm at f/2.2 (effective)
Sony Kit lens at 18mm, f/3.5
Canon FD 24mm at 100% crop
Sony Kit lens at 100% crop
To my eye, the Canon FD is sharper.