I think my online data will be relevant if this guy is as good as you say. His postflop sizing's are very good.
Your Kx/8x will be a 0EV bluff catcher OTR.
Here is UTG (6max so almost equivalent to UTG+2 in 9max). Triple Barrel Range
Underbluffed for all sizing's except B120 OTR which is GTOish
Below is a BTNvsBB triple barrel range to compare to filters.
Poker is all about nuance.
The 9x OTT gives a lot of his hands equity which makes me want to call down here.
Data point 1: Flush draw missed (over bluffed 36 weak vs 35 weak)---->Good data point to call down
(This is BTNvsBB SRP but the texture tendencies don't change based on position).
Data point 2: Middle/low pairing OTR on K-T high board (overbluffed 37 weak vs 35 weak)
Data point 3: Bet sizing indicates he has strong grasp of fundamentals----->Overbluff as a default
Counter points:
1. In theory you are supposed to RFI tighter when there is a straddle so it's possible his preflop range is slightly stronger than it should be. I don't think this point is strong enough to overturn the other 3 data points.
Conclusion: Call Down with 0EV Bluff Catchers (Any Kx/8x).
Interested in results.