InstaWallet doesn `t mix coins. SR does. Torwallet mix coins once a day fo free.
InstaWallet, like all hosted eWallets use a shared wallet for customer's deposits and withdrawals. As a result, it is unlikely your withdrawals will be the same coin as your withdrawal. It might be easy to know the coins went through instawallet, but without records from InstaWallet, it wouldn't be possible to know if the InstaWallet used to withdraw was the same one used for the deposit.