Metering Head XR3i

Can anyone tell me if it would ok to use XR3i metering head on 8v Gti.  I have seen a couple of posts where its been mentioned but very breifly.  The XR3i version's casing is made of alloy and the injectors that came with it had the same bosch part numbers as the my golfs..  I have put it on and it seems to run ok.  Reason for the change was water in the tank because of rusted filler neck.  

I have put just the fuel distributor on and it seems to run fine, the only problem I have now is that there seems to be a small missfire between 1000-2000 rpm and poor warm up idle.  At first it starts and tickover is high 1500-1600rpm for few mins, then it changes to low rpm <800 rpm until fully warm. Once engine is warm the idle is fine, the missfire is still there (very small) but the care pulls fine, and goes well.  I have adjusted the mixture (leaner) to try and iron out the missfire which reduced it a bit but it is still there.

Anybody with any thoughts on this would be appreciated.

Cussco