I disagree with Energizer based on my experience but at the same time a lot of it is down to driver skill and ability so it's a two fold answer.
It's not really the wheel that makes you faster, but the brakes. The brake pedal on the DFP/DFGT is so bad that it can make it harder to get a good feel for how much pressure you are applying. You just don't get any feel for what you are doing. Get a better pedal with more resistance and you can more consistantly apply the amount you want, on every corner. If you can get progressive resistance than even better. So, to give an example, I ran the 24 hours of Le Mans this past August, I started the first 18 hours with a DFP and around 18 I received a G27. Within 15 minutes of using it I beat my previous best lap by around 2 seconds and with no draft. By the end of the race it was 5 seconds. Sure, some of it was learning the track and adapting, but a lot was the brakes. I guess I needed that improved feedback from the pedal to get the most out of my ability. Either way, since getting the wheel my performance in the WRS has improved. Have my times improved since getting the T500 over the G27? I don't know, maybe a little, but not a huge leap like getting the G27, but my enjoyment while playing has gone up. I love the larger rim, the FFB, and I'm getting used to the static paddles.
Now, having said that, some of the fastest GT5 drivers in the world use DFGT. Those people are probably fast no matter what you give them, they will probably get the most out of whatever is in front of them. They may not see any improvement from one wheel to the other because they are already at the top of their game. At the same time, I'm willing to bet if you gave them a couple hours with a CSRE w/ CSP, G27, T500 etc. they would never go back to the DFGT. It's a nice wheel for sure, especially at the price, but the pedals hold it back and there is higher quality stuff out there.
So if you are looking to get a new wheel, I would examine what's out there, start checking out the sim racing hardware forum here on GTPlanet, watch reviews on insidesimracing and wherever else. Read WRSers signatures, PM them about their wheels. Most are very willing to help and I'm sure others that have gone from DFGT to different wheels will jump in and give you an opinion. Figure out what your complaints with the DFGT are, see if other wheels address those, and fall within your budget. I have my recommendations based on budget but I've never used Fanatec products so I can't answer directly for those but always hear very good things.
Sorry for the long winded answer, I guess what it comes down to is both energizer and I are right.
He may find that the wheel makes no difference, I found a huge difference in lap times and enjoyment. I hope it helps.👍