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... demonstrated in a completely artificial -regtest environment...
If you can Sybil somebody and control their view of the network, then it seems to me there are more potentially profitable attacks than "make them pay more in fees than they should."
But please feel free to demonstrate an actual, effective Sybil on the fee estimation code. bitcoincore.org is running a wallet-less bitcoind on port 8333 that generates the graphs at bitcoincore.org/smartfee/
(hacking into the web server to make it LOOK like the fee estimation code failed doesn't count, you have to manage to control it's p2p network connections and then manipulate the memory pool).