Why limit your target consumer base so much? Legends comprise only a small fraction of the players on here. That alone should tell you why we don't yet have legend-only drivers.
More realistic, but also probably doubtful, is to give each driver a range for its attributes (distance, forgiveness, spin, etc.), with the top of the range representing the current maximum for that driver. (ie: R11 driver with a range of 230 to 282 yds).
Using the above example....as a Hack, the R11 driver would average 230 yds, but as a legend, it would average 282 (only with less forgiveness). Advancing from one tier to the next, "unlocks" the attribute changes (extra distance, decreased forgiveness, etc.), which modify your existing driver only if you buy them as an "upgrade".
Your driver improves with each tier advancement, until it reaches its maximum at Legend.....thereby representing the real-life progression one would likely experience.
Let's face it...even using an identical driver as him, there ain't many folks on here that are gonna hit it as far as Bubba Watson, right? At least, not unless they've put a similar amount of time, effort, and money, into learning how to accomplish exactly that.
Just a thought....