I am seeing two potential issues though...
I'm not sure I follow you. From what OP has said, his outputs are already in Whirlpool:
If you have a wallet with say >20 UTXOs that were previously mixed using Whirlpool (resulting in equal amounts), how can you best move then to another wallet.
Given that, and given he wants to move them from Whirlpool to another wallet, then the "mix to external xpub" option is the best option.
Another thing is why you want to move to another wallet?
To move your Whirlpooled outputs directly to cold storage. Once I have a UTXO which has had 20+ free remixes and I don't intend to spend anytime soon, I gain very little by letting it sit on a hot wallet an accumulate even more free remixes. Using this method I can move this UTXO to a cold storage wallet, increase its security, and also redirect any free remixes to other UTXOs of the same size in that wallet instead.
If there is not need to touch that balance, then is better to wait for low fees.
The fees do not matter given my suggestion above. Whirlpool remixes are free. Mixing to an external xpub is also free, since it is the new entrants to each round which pay the fee. Mixing to an external wallet is exactly the same as just accumulating more free remixes.