Some guys in a different thread mentioned that the design of the gearing in a DFGT differs from that in the DFP (as you'd expect, of course) and apart from it being smoother and quieter, there's a more linear response. I used to feel like my DFP was easy to turn from the centre but towards the limit of rotation is got stiffer and stiffer, so although I can't remember noticing when I last used a DFP, I suppose they meant it was easier to turn throughout the rotation range, which would make you faster. However, geared wheels will never be as linear in that respect as a belted one like the T500RS, but the DFGT is a great wheel if you're thinking of upgrading... That said, if you're not fussy or overly sensitive, you might find the DFGT isn't a big enough upgrade. It's quieter, has a bigger wheel, is smoother and has some GT5-specific controls, but uses the same pedals, same shifters and is still made of plastic.
That's why I chose the G27 after my DFP, personally I didn't think the DFGT was a big enough upgrade for the price difference between it and a G27, as in if I'd bought the DFGT I'm pretty sure I would've wanted to upgrade to the G27 soon after so it would've been a waste. I really wanted metal, a clutch pedal and an H-shifter, though, your priorities may be different.
Oh and my lap times definitely improved when I got the G27, the brake pedal alone made a huge difference, as did the smoothness of the steering. The fact that the reverse gear is much easier to engage really helps, too, but if you need to reverse during a lap it's probably not going to be a fast one... It helps to recover more quickly during a race though.