If you raise the height of the front derailleur to accomodate the 55t
ring, then you will drag the chain on the bottom of the front
derailleur, especially an Ultegra, if you use a 24 or 26t granny. If
you want 55/42, you will be best off getting a Dura Ace Triple front
derailleur. It's cage is longer than the Ultegra.
There are so few times when you spin out a 53/11 at 100-120 rpm. In
most of those cases, you'd be better off not pedaling and getting in a
more aerodynamic tuck. Certainly, you won't be cornering at those
speeds. You don't gain much in going from a 53 to 55 and it is going
to cost big$.
Practice spinning faster.