Just executed a £100 transfer to a USD bank account using the amex fx facility. Rate was 1.5664. HSBC business banking was quoting 1.5529 about 15 mins later, so probably a good enough rate (HSBC business banking charges £17 fee)

note that when sending to USA based banks, you don't need an IBAN, in fact according to the HSBC USA FAQ, IBANs are not used in the US. You need your routing code and account number. For those with US cheque books, the routing number is the 9 digits that preceed your account number at the bottom of the cheque.

Used debit card to fund the transfer,so total fees were £10.