I personally thought it was 54, with the exception of certain spells that can cast at 56 in certain situations.
Magic missile for vokers?
The spell cap is lvl 55. Not 56, not 60 - its lvl 55 for mortals. This was done in 1.0 after Firular found a way to cast at lvl 60 for some strange reason.
53 is the lvl any prepared lvl 50 invoker casts at to my knowledge. Roll one and work out how yourself.
In 1.0 is was lvl 54. It changed in 2.0 and I don't believe has been changed back.
L-A