I've just done 5 clean laps in GT3 PAL and GT4 Japanese demo using a RX-7 Savanna car with the Dualshock2, MT, all aids off. The average times are as follows:
GT3: 1'47.024 (from 1'48.521 to 1'45.195)
GT4D: 1'47.318 (from 1'47.500 to 1'46.001)
In GT3, the Savanna isn't featured in arcade mode so I had to buy it. When bought, it had 202 PS/6500rpm and a torque of 28 kg f.m/3500rpm. The GT4D Savanna has 205 PS and 27.50 kg f.m.
In order to boost the HP of the GT3 Savanna I changed the oil, only to boost the power all the way to 212HP. Oops! The tyre choice in GT4D doesn't tell us exactely what kind of rubber it is so I used the default tyres in GT3 and the Sports in GT4D. Not the best choice, I know, but it felt okay since Racing in GT4 must be some sort of soft tyre and Street is more or less GT3 Simulation.
Anyway, what's most interesting is that the handling in GT3 felt artificial and too simple, as opposed to GT4. Also, the brakes were a bit too effective in GT3, something I'm glad they've tweaked for GT4.
Brakes have always been too effective in the GT games IMO.