amd64 images use BIOS or UEFI. Macs use a variant of EFI (an earlier version of what's now called UEFI), they apparently can't cope with multi-catalog CDs and the MAC variant images fix this.
I based mine on some else his blog as well and open sourced mine as well in hope others benefit from it. I am glad to see most jekyll/middleman/octopress users open source their blog. Without it, mine wouldn't look as good as it does. What I have added is a way to turn jekyll a bit more into a CMS by automating the generation and deployment: http://born2code.net/blog/2013/05/27/simplify-your-jekyll-pu...
My takeaway is, that the reason that this is possible is that they care about data structure. A language can give you an order of magnitude performance, but - according to Ville - you can almost get infinitive improvement if you rethink the algorithms and data structure.
This is an interesting scenario. I've resolved this by running the entire build pipeline on wercker and only deploy the binary on Heroku by using the null buildpack. I'll write up a post on this in the coming days.