It is definitely worth doing. The exhaust side of a ga16de head is of a horrible ratio compared to the intake side. Not sure if it will do you much good with the stock, extremely crappy, exhaust manifold though.
You won't see any HP gains from the TB, but everyone swears it helps a ton with throttle response. In order to feel a gain from a larger TB, you would have to upgrade your MAF most likely, and in order to do that you have to have your ECU reprogrammed by someone like Jim Wolf Technology ($600). I have never heard of anyone porting the TB, but lots of people upgrade to larger ones from different cars. Some of the people upgraded their TB in the threads below.
Here is a heap of info regarding porting a ga16de head. I know you have to be very careful because there are some water passages you might accidentally go through and then the head is ruined, also if you make the passages too thin they can crack when they got real hot:
My fav is sentra.net. Click Engine under Kojima's Garage, and find the head porting: Sentra.net - Resource for Nissan Sentra, 200SX, and NX Coupe Vehicles
I think all 4 of these builds talk about porting: Modified - 200SX 1.6
and this guy even did flow tests: The Ga16de Thread.
Might be some porting talk in these two build threads:
The Official GA16 Forum - Recent Blogs Posts - Blogs