I have heard of other people (in DIY section) that add pga or vodka to their juices for a better throat hit. LeCig carries greencig carts for $12 but they dont fit all kr8's so you would need to verify that they will fit your model first if you decide to try them.
I have the regular cartomizers too but cant really tell a difference compared to GreenCigs in throat hit. I get a good TH from my higher nic juice and Atomic fireball juice. I havent tried menthol juice yet but I heard it gives a good TH.