To actually answer the question at hand here....Yes, it would be a fairly significant upgrade. More tubes, upgraded caps, two power supplies (dual mono design, balanced outputs. Also has phase control, a useless feature for me personally, but an addition nonetheless.
Sonically the first you notice between the two is the bass is considerably better with the 5.5, and the notes decay more noticeably. Where the 2.5 will simply play a string of notes (very nicely I might ad); with the 5.5 you know exactly where they begin and end, as well as what they are. Instrument placement was a little better as well, which could just mean an adjustment to the soundstage. IE front to back, left to right.
Its just simply a stronger preamp all around. Not that the 2.5 is bad, it is quite excellent and probably presents a better overall "value", but technically and sonically the 5.5 is better. Whether it is worth the extra money to some is subjective, whether it is better preamp is unquestionable.