Short answer is yes you do get a faster fermentation in a 1 gallon batch.
Long answer is there are two reasons for using the whole package.
Firstly, what you are adding (typically 5 grams) is not a fixed quantity of yeast needed to ferment your wine so much as a starter culture of yeast. The yeast in the packet represents the minimum amount of culture recommend to start an active fermentation, regardless of batch size.
When adding a packet of yeast to 5 gallons of wine, the yeast will typically multiply to around 100 to 150 times what you put in. In the case of a one gallon batch of wine, the yeast will multiply, but not as many times as it does when pitched into a larger batch, therefore it will get to the required levels faster. The yeast will only reproduce itself into great enough numbers to complete the job that has been placed before it.
So, when you add a whole packet of yeast to 1 gallon of wine, you are not adding too much yeast. You are simply adding the minimum amount required to start an active fermentation. To add less then a package will result in a slow starting fermentation that will take extra time to finish the job. A single yeast cell could reproduce itself enough times to ferment 5 gallons of wine but it would take an awfully long time.
Secondly, there would be the issue of what to do with the rest of the yeast anyway. Unlike yeast you buy for breadmaking, these packets of yeast are packaged under "sterile" not "food-grade" conditions, and are sealed with nitrogen to maintain this sterile level while in the package.
Once they are opened they are no longer sterile. The seal has been compromised. So, storing an opened package of yeast for any length of time is really not a viable idea, particularly when you weigh it against the costs of the packet.
