when i converted my 3i from a series 2 i just unplugged everything from the loom in the turbo and threaded it out of the car via the fuse box hole (thread it into the engine bay) took half an hour or so with a mate helping to feed it through but worked fine for us, same method fitting it back into the 3i. i wouldnt cut the loom about if you can avoid it mate.
also rewiring it isnt as hard as it looks, we just took time to label where each plug went and take photos etc. hope this helps