That lense is f5.6 on the long end IIRC, and the extenders cost you 2 stops which brings you to an f7 at your best. That's gonna means shooting in nothing but the brightest conditions and paying a serious price in your depth of field.
Furthermore, I don't think it's actually compatible with the extenders to begin with - I'll fit on alright, but I'm reasonably certain that you'll loose metering and autofocus so you have to do everything on manual (but you should be doing that anyway ).
This is why I tend to recommend the 70-200 IS lense - it's a helluva lot more money, but it gives you