GC benchmarks for trunk vs gc-compact seem to be about the same:
$ make benchmark ITEM=gc
./revision.h unchanged
/Users/aaron/.rbenv/shims/ruby --disable=gems -rrubygems -I./benchmark/lib ./benchmark/benchmark-driver/exe/benchmark-driver \
--executables="compare-ruby::/Users/aaron/.rbenv/shims/ruby --disable=gems -I.ext/common --disable-gem" \
--executables="built-ruby::./miniruby -I./lib -I. -I.ext/common -r./prelude --disable-gem" \