Yep, around 40% speed boost for me too.
Test scene with 1280 Samples:
Only GPU - 8min 53sec
GPU+CPU - 5min 29sec
WIN 7
750TI
Xeon E3 1230 4x3.3GHz
Tilesize Test:
128 Samples
64 Tiles = 39sec
16 Tiles = 55sec
Same Scene only change Samples to 1280:
64 Tiles = 5min 45sec
16 Tiles = 5min 29sec