Gaudio was talented but winning big clay titles for him wasn't exactly underachieving, sure he had Coria, JCF, Guga and Moya among a few others hanging around during his peak clay years but on his day he could do unthinkable things on a clay court.
Calleri if we're talking about underachiever on a particular surface (clay), that guy could get red hot about twice a year but when he did he was explosive (JCF in the DC in Spain, Hewitt at the USO, Chela at Kitzi)
Yes, Calleri when he was on was could hit winners from all over the place. In that match against Hewitt he hit like 70 winners. It was insane.
He also destroyed Agassi at Miami in 2004 http://www.youtube.com/watch?v=2xWwocRdlsI
Another fantastic performance.