I did as suggested, and contacted Mirage, and got a very helpful response from them. It seems that to get the Edit part to fit correctly you need to add 4 x 0.5mm shims (2 for the suspension shafts, and 2 for the screws)
The HPI equivalent hop-up Just Fits (TM)
I have also hit another problem with one of the Edit parts, this time it's the Firestorm Alloy Knuckle Arms. They don't fit behind the stock HPI wheels, it would seem (the ends of the "steering arm" rubs against the rim to the point where it stops it turning when the wheel nut is tightened)!
I've asked Mirage's opinion, so we'll see what they come back with, as maybe I've goofed somewhere (but I can't see where from the diagrams). I've been very impressed so far with the help from Mirage, but I must say I've gone from being quite impressed with the perceived quality on fitting the Edit parts to being pretty disappointed that they don't seem to really fit properly.