I came from Logitech so I can't comment on the move from T300, but overall it was a huge step up from the gear-driven wheel. The directness and fidelity of the DD wheel communicates the FFB in a more intuitive, consistent manner. I am using the boost kit, not because I like the wheel strong (I prefer a lighter, informative wheel) but rather because the added boost increases the dynamic range of the wheel, allowing it to transfer more sensations even if not set to an overly high strength.
As a whole the move to DD has made me more consistent and confident, but not really any faster. Of course my overall performance has improved greatly because of that extra confidence in the car, which to me was the best part of the move to DD: being able to consistently feel the car and push the limit in a more comfortable way.
The FFB in ACC and WRC are just incredible with the DD. A huge step up from the G29. GT7, well, what can you say, I don't think anything will make the FFB in Gran Turismo better. That said, the same directness and fidelity I mentioned above has also helped me to be a better driver in GT7 compared to GTS using the G29. However, given that the GT DD Pro is the official wheel of GT7, it's a huge disappointment. The FFB is informative enough, but dull and lacking in general as we have been accustomed to from Polyphony. So in GT7 it's an improvement but overall a big disappointment. ACC and WRC it's been a huge improvement. and I am very happy I made the upgrade.