I was in a similar position to you nearly 3 years ago. I'd a Focus ST3 and was looking to purchase a GTD. I was fed up with the big fuel/road tax bills on the ST.
I went for a GTD. Whilst it has been an amazing car, it's not in the same league performance-wise of the ST. The GTD isn't slow by any means, but it delivers its power in a different way to the ST. There's not much go in first and second gears, but the car is deceptively rapid in third and picks up speed very smoothly.
One thing I don;t like about the GTD and probably diesels in general is that if you aren't in the torque band, then the car isn't going to accelerate. The ST in many ways made me lazy with there being so much torque in any gear at any revs. You just press the throttle with your foot and the ST would be off.
Plus sides, the GTD is very economical. I'm getting 450 miles per tank as opposed to 260 miles in the Focus and that had a 60 litre tank if I remember correctly.
Also, road tax for my 3 door manual is £20 per year as opposed to £285 for the Focus.
It's a really great car, but you will notice the performance decrease compared to the ST. I'm probably going to go GTI or R when I change next. I do less than 9k miles per year, so it makes more sense.
Hope this helps!