Definitely worth buying parts instead of a complete bike. The duty is calculated to the price plus postage then the VAT is calculated based on that total. Duty on parts is 4.7% whereas on whole bikes it's 14%.
Also, I just imported the frame, headset and forks. I could get the other bits cheaper by shopping around and negotiating here or from Germany.
And bear in mind that you can normally negotiate a discount off the RRP with the US dealer too.
All in all I now have a bike I couldn't have justified at UK prices but am also aware of potential warranty problems in that it might need to be sent back to the US.
Hope this helps.