agg23 posted his G3 numbers, but by and large for G3s and G4s it's about 2x speed. My father's 1.5GHz G4 mini, for example, benches at 6185ms without it but 2811ms with it in SunSpider.
For G5s, however, it seems to hit a wall with the extra hardware. SunSpider actually takes a hit, but with tuning to blacklist expensive JSOPs, Dromaeo and V8 both get wins. It's not as dramatic, however, and a very fast G4 will actually bench faster. I'm trying to figure out why; it seems that the G5's longer pipeline takes a bigger penalty for stalls. That said, the base interpreter without acceleration on my quad is around 3300ms. The accelerator "accelerates" that to 3700ms, but Dromaeo gains several runs/sec and the V8 score jumps, so I'm calling it an overall win.
For G5s, however, it seems to hit a wall with the extra hardware. SunSpider actually takes a hit, but with tuning to blacklist expensive JSOPs, Dromaeo and V8 both get wins. It's not as dramatic, however, and a very fast G4 will actually bench faster. I'm trying to figure out why; it seems that the G5's longer pipeline takes a bigger penalty for stalls. That said, the base interpreter without acceleration on my quad is around 3300ms. The accelerator "accelerates" that to 3700ms, but Dromaeo gains several runs/sec and the V8 score jumps, so I'm calling it an overall win.