I'd say go ahead and do it, if the hassle and cost of having a competent Al welder, preferably a frambuilder do it are worth it for you. I've had my steerer tube cut and welded (conversion from quill to threadless system) myself. It came out looking about like an un-cut tube and it has held up fine so far.

It cost me the grand sum of $10 but that was steel so it'll cost you a lot more $$$.

Don't do the extender thing. Heavy, nasty and probably flexy and creak-y as well, plus there's the tendency of quills getting stuck in there, and then how do you preload the headset?