Performance isn't necessarily better.
In regards to the specific question of whether a pair of drives in RAID 0 can degrade performance as compared to a single identical drive, other than the unlikely CPU example above the answer is "probably not." Some may say that access time can be increased (worse) due to a specific read (or write) requiring both heads to seek to their portion of the data, creating the result that average access time will be slower. However, this only really applies when accessing large numbers of small files as larger files will benefit from the improved STR. Unless the configured stripe size is too small, most of the small reads will access only one drive.
I encourage the OP to read the storage review FAQ I posted to decide whether RAID 0 will actually be beneficial. Like I said, it really isn't a good idea unless you do stuff like video or audio editing. If you get the second 250GB drive, it might be more advantageous to run it as an independent drive rather than in RAID.