Shane, listen to that little voice in your head that keeps saying 'get the Sigma 150 Macro', but do some serious research before deciding on the 2X T/C in favour of the 1.4X T/C.
Comparing any 18 - ?mm lens with the Sigma 150mm is like comparing a VW with a Ferrari. They will both get you to where you want to go, and the VW will deliver you, but the Ferrari will put a smile on your face.