Interesting question, but I guess a key element to consider when deciding if this is fair or not, is the fact that we are talking about public funds, so stealing the money that other hard working people have paid in tax.
When you consider the main objective as preventing stealing from the state purse, then I don't see a problem with giving the whistleblower 5% of the amount.
If a whistleblower was truly involved in the theft, then his cut would have to be under 5% to make whistleblowing a lucrative option for him.
Very interesting point, though the percentage varies from 2.5% to 5%......but a case actually made me post this; Senator blew whistle on another sets of senators cos they didn't give him shares from a looted sum.