Pass. With an XS frame many parts may not fit (fork, seatpost), and you alway run into incidental costs that add up over the long run (cables, housing,etc.)

It's been my experience that you should just save up your money to buy a new bike (on sale) a year or so down the road rather than continually dumping money into the components of your current bike.
Just ride your GT until it's dust, learn to keep it running smoothly, and save your pennies until you can afford the "new expensive stuff".

That's not to say that I actually do that, but I think it's good advice.