just MOTd my non turbo standard R32, only mod i have is a magnex exhaust my CO2 = 0.30, limit is... 0.30!!! pretty close, but a pass...!
but rest of the emissions wernt 'too' bad...
looks like you is probably gonna get caned!
I have just fited an ecotec valve... they are supposed to help reduce emmisions by about 20% or so... im going to have the emmisions tested again next week out of curiosity...
Assuming that your car is reasonably tuned (ie. not running too rich) if you put the CAT back in, then it will probably pass without too much hassle.
If the MOT examiners facility has a newísh emissions testing machine then it will have parameters for the Skyline (my tester did). Don't let on about the timing pickup on the ignition amplifier, then they need to hold the revs manually, which can allow a little bit of 'operator adjustment' as the machine is not monitoring it.
Maybe if you know the tester, he will let you run the emissions test first, before going over the car. This will allow you to take it away and fit the CAT if you need too.