Hey there, I'm 5'6.5 and have a short inseam of 30 inches +/- 1 inch. The inseam is so hard to measure and your flexibility etc etc all make it a variable length.
Anyway, a bike store sized me wrong once to a 52cm, and then I went down to 50cm. Even then it wasn't feeling all that good. Finally, I went to another store and I tried out the compact frame and now I'm more than happy. I have the long torso, short legs thing a ma jig and I think that's what Giant's compact geometry is good for, I come to realize I don't fit too well for traditional frames.
Now if your current medium frame is too long, that could be cuz most Giants have longer top tubes, to accomdate the compact geo. Assuming your seat is positioned correctly, seat height correct etc etc, try slapping on a 90mm stem. It will "increase" the responsiveness but it maybe more comfy. If not, I would consider trying a small frame and start to see which stem length, position etc will fit.