"So if I lift the entire car a 1/2" or so how will this change the handling?"
OK -- so you want to raise the car because of the poor street conditions -- that's all good.
When you raise the H&R's you will want to find a very good Porsche alignment shop/tech to reset both front and rear. If you can find a guy in Mass with a Biesbarth German alignment machine they may be your best bet to tune out some (but not all) of the understeer during your twisty road runs. If you keep the same tires - sound like good rubber - then you will always have some understeer (and that is a 'safe' thing).
You may not like the same alignment for the normal street/road driving. If you put a little front toe out in the alignment the car will turn into turns quicker but will not be very steady down the straight roads at high speed -- this is all a trade off
IMHO of course...
Edit: Also, if you put a GT3 front sway bar in the Boxster that may help also -- some folks do this for Ax, etc.