No. if it's floating, it's not correctly railed. Hornby steam locos can take the tightest curves at high speed if railed correctly.
I believe that "floating" in this instance is referring to a centre pair of driving wheels that allows for lateral movement within the frames. The other option would be to have a flangeless pair of centre drivers.
I'm only familiar with 00 gauge modelling, rather than 0, but it doesn't seem like either of these is a common feature on the ready to run six-coupled models (notably LNER Pacifics) available from Hornby or Bachmann which, as you say, can handle pretty much all set track curves.
The radius of an 0 gauge second radius curve doesn't seem restrictive in relation to its' 00 gauge equivalent, so I wouldn't have thought that it would prove a problem having all driving wheels flanged,: Looking at kit built 0 gauge A2s they appear to have fully flanged driving wheels: However that's not to say that either of the options outlined above wouldn't allow for that extra level of movement required to negotiate these curves.
Alternatively,
bogieman, is there sufficient lateral (side to side, just to be clear to all) movement available in the leading bogie? There isn't any stiffness there in turning to left or right to a sufficient degree, and the bogie - wheelsets or bogie frame - isn't fouling any aspects of the main frame, or the cylinders, on corners at all? This can have quite a marked effect on running qualities on the real railways, as much as on our small ones!