John999,
Does your beneficairy have their account with a bank other than HSBC?
The reason I ask is that when transferring funds to or from the USA, a charge is often made by an intermediary bank through which the transfer is routed. This may explain the missing $25 and HSBC should have been able to explain that if it was the case.
I make transfers and payments to the US on a fairly frequent basis and have Sterling and US Dollar current accounts held in London and also a US Dollar account held in the USA.
I transfer money (up to £10k per day is allowable) from Citi UK to Citi US accounts and then use their electronic cheque system at Citi US which deposits the money in the other person's account within 3 days. Both the Citi-Citi transfer and the electronic cheque transfer are Free to use and wil leave more money in your pocket.
There is no need to close your HSBC account but it may be worth thinking about opening the Citibank accounts, the USA account maybe a little tougher to open from over here because of the money laundering regs but I would think that providing certified copies of a passport and bank statement should be enough or maybe Citi UK may help you with it.
Also as an afterthought, I think that HSBC help with opening accounts in HSBC Branches in the USA but that maybe just for Premier account users.