Finally got the performance I should have!
I have always felt that there is just too much variability in the performance of E55's based on anecdotal evidence from the E55's on this forum. For years, I have felt my car was below average, especially after the reflash fiasco a few years back. The K1 mod improved things some. The retune has improved things considerably more. While I know the larger pulley should be the major reason for the improvement, I feel the tune is even more important if your baseline is a poor tune to begin with, which is where I was before I did the K1mod.
I am sure my experience is not unique and that there are many E55 owners out there whose cars are performing well below average because of a bad tune. If you think you are one of them, I would strongly recommend investing in a new tune, even if you don't want to make any other changes. It is a travesty to own one of these beasts at 80% of its potential.