To answer your question, yes, you can add onto your gpt partition (and you are limited to 2TB so long as you SP1 installed), however, I have never done this, but I also have a 2TB gpt partition on a raid 5 and I'm going to experiment and add another hd soon to see how it goes (I always cringe when expanding a partition table, however, due to GPT's redundancy it would seem safer to do it on this table than others...maybe).
As for performance of multiple partitions, I'm not too sure. Since you're talking about 2TB, I'm sure you have some type of array going on so there would be a number of factors to involve other than simply the # of partitions. I would also say it depends on what you're using your array for and the types of files and sizes of these files.
Nate