Define ´sway´.Anyhow, while in the 90 mph range, I do feel a little sway,
Sway bars, aka anti roll bars only counteract body roll.
Straight line instability can have several causes.
Wallowing/´floating´ through corners too.
Lastly, the concept of a mid engine can appéar instable at higher speeds. This again can be tackled several ways.
The chin lip mentioned is a very simple one which reduces the lift under the front (see photo) and with that a good part of the ´light´ feeling.
Oh and OEM wheels, tyre specs are best for the car.