I think that it could be a simple change, I'll check it out. As of right now, you can create a rake task for 'test:prepare' so that it doesn't break.

@Sapherous This might be a little late for you, but I just went through this exercise myself, mostly based on these instructions:
http://tekin.co.uk/2014/09/performance-test-rails-against-real-data/
I'm running rails 4.2.4

I modified the author's rake task to look like this:

# lib/tasks/test_benchmark.rake
# Use this rake task to run performance tests against the "benchmark"
# environment. See test/benchmark_helper.rb for more details.
namespace :test do
  Rake::TestTask.new(:real_world_benchmark => ['test:perftest:benchmark_mode', 'test:real_world_profile']) do |t|
  end

  Rake::TestTask.new(:real_world_profile) do |t|
    t.libs << 'test'
    t.pattern = 'test/performance/**/*_test.rb'
  end
end

Otherwise, most of the other setup worked as is, and I can now run bundle exec rake test:real_world_benchmark against my development database.

One other gotcha I ran into was that I had to add :benchmark to the [:development, :test] group in myGemfileand make sure that rails-perfest and ruby-prof were included.