Madman, if your ambient temp is 20 degrees C and you are running stock turbos, forget about the HKS intercooler! The larger HKS IC would just make the charge pressure drop and along with that , your horse power.
I was running 1.02 bar on my stock turbos before. But ambient temp here is around 35 C. So, with a view towards high horses later, I put in the HKS GT IC. Absolutely no power gain! But of course the GT IC is needed for my current 2540s.
I am always against altering the stock turbos as I fear it won't be reliable. If you just change the impellers to steel, that sounds okay to me and what that does is, it enables youn to run more boost than the stock turbos. But if you start altering the turbo housing in the process, that would result in a thinner housing and in my climate, that might induce crack in the housing. If you want better performance up to between 450-500 horses without any substantial increase in lag, I would recommend the 2510 and the metal head gasket. With all your current mods, especially the cams and pulley, the 2510 would really make full use of them.
That's my 2 cents worth.