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Yep, you are certainly right to a degree, however I know the on one parkwood has been out a few years, and the 27+ version is a new addition to their range. 29ers with big clearances can squeeze the full 3.0 tyres, but generally better to get one designed for the job.
Then your problem is that a lot of new 27+ frames will be using the new 148 boost spacing and therefore awkward for swapping wheelsets.
Yep, all frame dependent. I'm looking into this at the mo too. For example the on one parkwood 29er is sold as both a 29er and 27+ on the same frame. Compare that to the 29er titus el guapo (also basically an on one) and you've got no chance getting any thing plus sized between the chainstays.
Need to get a measuring tape out...