One other (possible) issue I'm seeing with ECU selection is that the OP's car is an auto (like mine), from reading some old threads not many aftermarket ECUs works properly with auto Skylines.
Something to do with that the stock ECU backs off timing when the auto box changes gears, however this feather is not available on any of the aftermarket stand along ECU (as far as I know), so you will get very harsh changes if you remove the stock ECU.
Not sure how it is now, as I said my understanding is pretty dated...
I know that PFC doesn't properly work with auto skylines, does Nistune work 'properly' with autos?
Last time I talked to Andrew from Hyper drive (few years back), he reckons other than converting to manual if I wanted to keep the auto box I need to get Emange Ultimate which is still a piggy back. In the end I just got a SAFC 2 since I wasn't planning on modding it further for the time being.
But I recall one of the WA mods having an auto Stagea & running a Nistune...