That is surely a 531 framed bike... I believe that I have described these in the past as quite a number have passed through my hands and I know I have described them as being noodley.

With that being said, my friend built one up into a fixed gear and despite the massive power he can lay down he couldn't break it... the frame flex might become more noticeable with a derailer set up as I am sure it could cause a little chain rub.

The frame and fork are quite light with a bunch of really weighty parts hung on them... the Crane derailers are nice.

The bikes themselves have a rather nice ride quality too.