IIRC, the IG chain is just slightly narrower than the HG.
I had an 8s IG setup for years on my mtn bike before I upgraded it to 9s.
I used to use SRAM 8s Powerlinks with Shimano IG chains on it, and I've read that would not have worked with the HG chain.

A little info about the IG here: http://www.sheldonbrown.com/k7.html#interactive
I believe only the chain & cassette were different on IG, but the center-to-center cog spacing was the same.