I can't tell you how hard it is, but I'm taking a similar route at the moment. I did my PPL in the UK, and am doing the JAA ATPL theory exams at the moment. I plan on going out to the USA to do my CPL/IR early next year once I've finished the theory (JAA and studying FAA).
The question is how much of a hurry you're in - given the current job market, I'm in no great rush to complete my training. One of the great advantages to me of FAA training is the chance to spend a few months in some cool new places, make some new friends and have some serious fun doing it.
If you're working to pay for your training, you aren't realistically going to manage it in less than 2 years or so even if you use all your holiday time flying so don't be in a rush - enjoy your flying.
The US guys on this forum offer good advice (which I've benefited from recently), so take heed.