As an interloper from the other side (but possibly just the other side of town) I might mention that the Oneworld Explorer RTW doesn't have any mileage limits, just segment count and per-continent limits. It might work better for you given you'd like to do some surface segments or a bunch of open-jaws.

One other element to consider about a premium-class ticket instead of coach is the ability to accrue a ton of miles for future use. On the OWE for instance, with elite bonuses, the lack of a mileage limit means you can usually leverage an award RTW out of a paid one. Probably hard to achieve the same on a Star, Skyteam or single/multi-line RTW (like UA/EK) but the cost/benefit ratio is still worth considering when you think about spreading the cost differential (between Y and J) over the course of a year's travels.