Personal experience (as always, YMMV) says stretching a too small bike is opening a can of worms that is actually totally avoidable, at least with a C&V road bike.
Get one that fits right, right from the start, and save yourself a lot of aggravation.
Once I start messing around with an extended seat post, I'm in real trouble. I usually then end up having to get a modified stem, and still have reach problems, anyways.
Someone else mentioned it- the handling on a bike so modified can get strange and weird...
I've also found that such radically altered bicycle geometry, for me at least, actually makes the bike harder to pedal.
It may be worth it to go to a high end bike shop to get your self professionally sized (if you can find one that still offers that service).
MTB's are incredibly irritating in this respect, especially with their leaned back seat tubes, and lack of frame sizes above 18" for affordable dept. store bikes.
Good luck with the quest!

-D.S.