Hi Manxie
The exhaust is a straight swap with no need to change anything. There is a lambda sensor elimination kit but to be honest it makes hardly any difference if you do fit it. The only caution I have on exhausts is that Hitchcocks silencers are very expensive and the quality is dubious on the chroming. Have a look around to see what else is available.
The efi is reliable and although I have fitted a carb conversion just for shits and giggles does actually do a good job. The one Achilles heel is the fuel pump, reliable but if you do get a problem you will need nearly £400 to replace it which is ludicrous. Having said that my efi system was good enough for 20,000 miles with no problems before I fitted the carb.
The trick with Enfields is to simply enjoy them for what they are and don't try to tart them up too much. I have made this mistake with The Mighty Enfield and it is fighting the buggery out of me to get it back to one, reliable piece. If I could go back I would have changed the silencer and left it at that.
Ian