The instability of short bows is due to their overall design. Yes, their lightweight plays a part in it but so does their short length. Short bows have a smaller distribution of mass weight. This makes it easy to cant them either clockwise or counterclockwise at full draw. Canting even a few degrees with a short bow can result in a miss of a few inches at 30-40 yards. Short bows also have a sharper string angle at full draw thus making it difficult for some folks to get proper anchor positions. It can also lead to nock pinch if you do not compensate for it.

If you are looking for something a bit more stable then the Hoyt Xtec is probably a better choice. I have one in my possession and I will admit that I can get better accuracy out of it at 30-40 yards than I can with the Vipertec I owned. If you are not stuck on the Hoyt lineup then I suggest some of the new Bowtechs. The Patriot single cam and the Liberty are two sweet bows that fit everyone of your criteria.

1. Both are extremely quiet with the hush kit factory installed.
2. The Freedom cam on the Liberty is arguably the smoothest fast cam on the market
3. The Patriot SC can probably put you there with relative ease (depending on your setup) considering it has a 320-328 IBO speed rating with a 7 inch brace height.

Hope this helps.