The IMSA website gives ballparks of 525-530 for the GTLM class, and 500 for GTD.
The IMSA BoP (here's a
link to a random BoP table for a race this year) suggests that the Ferrari has significantly less boost in GTD format than it does GTLM. And that's the same car, with a conversion package. Also heavier.
So I'm not sure that the claim of GTD being more powerful than GTLM is true.