I had the same hard steering issue with USI SPX skis last year. The solution that I found came from Bergstroms carbides. I had a long, and very informative, talk with the Bergstroms folks. The problem, they said, is that the skis are not properly balanced. The solution is to use the shim that they sent me along with their carbides. The shim is placed beneath the back of the rubber ski grommet (?) between it and the ski itself. This places more weight on the back of the ski and makes it easier to turn. Doo a search on this site and you will find a lot more info on the subject.
The USI's are good skis but the Bergstrom carbides and shims definately improve the hard steering issue much more than any fix that USI suggested. Just my 02.
Good luck.