I was wondering if I add a complete pack of Wyeast to a 2.5 gallon batch rather than a 5 gallon batch what would happen. Would I "overyeast" the beer?
No. In fact, depending on the gravity, a smack pack might be just the "right" amount of yeast for 2.5 gallons.
The Wyeast Smack packs have a "minimum of 100 billion cells in a yeast slurry", but by the time they get to you commercially, some of those cells have died off. Commercial pitch rates are about 0.75 million cells/mL/P for ales (a bit higher for lagers) ... for a ~2.5 gallons (lets say 10000 mL) of 1.050 (12.5°P) wort, that works out to 125 billion cells. So already the smack pack is under-pitching, even for 2.5 gallons.
It is possible to over pitch
For most styles more yeast or even a full pitch* won't adversely effect the beer.
However if your beer style relies on yeast esters, this is where an over pitch can cause issues of low ester production making a beer miss its style goals. An over pitch will limit cell growth phase and in turn limit desired esters.
- Full Pitch: A massive cell count that doesn't require any growth in fermentation to complete the job