How is the performance of GDC vs DMD? I ran some simple benchmarks
that showed DMD significantly faster but I'm wondering what other people  have
found.

I suppose I should retract my original statement. I ran some more rough benchmarks
using code from http://shootout.alioth.debian.org/
(Source is given when I could find the link -- source files can also be downloaded
from the site as a package)

Here are some of the results:

Ackermann
- GDC 2.6X faster, 19% less memory

binary trees -http://shootout.alioth.debian.org/debian/benchmark.php?test=binarytrees&lang=dlang&id=0 - DMD 5X faster, GDC 38% less memory

fibonacci
- GDC 15% faster and 11% less memory

matrix
- GDC 2X faster, GDC 12% less memory

recursive -Source:http://shootout.alioth.debian.org/debian/benchmark.php?test=recursive&lang=dlang&id=0 - GDC 1.8X faster and 12.5% less memory

partial sums -Source:http://shootout.alioth.debian.org/gp4sandbox/benchmark.php?test=partialsums&lang=dlang - GDC 25% faster and 10% less memory

method calls
- DMD 2.6X faster, GDC 12% less memory

dispatch
- DMD 16% faster, GDC 12% less memory