Currently, only way to double iridium gain is having a diamond silktouch enchanted pickaxe, rock cutter does NOT silktouch iridium ore as it doesnt have strength of a diamond tool.
Tested on newest FTB pack, just don't know if is already fixed or not.
Btw, gregtech pyrite ore in the nether is awesome. Gives iron dust (after centrifuged) and tons of exp. Iridium ore in the nether is "easy" to notice over distance as it is a gray looking ore.