Jeff Lewis -
While not commonplace, a significant number of GB&I courses do have USGA slope ratings.
Jason Topp -
You are right about windy/rainy/cold conditions effecting scoring on links courses that can render conventional course ratings/slopes meaningless. However, after a competition is completed, it is common for the the equivalent of the course rating/slope (SSI - standard scratch index) to be adjusted based on how the field performed overall based on the conditions. If a course has an SSI of 71, but the scores come in much higher than "normal" based on adverse playing conditions, the SSI for that day might be 73 or 74 and handicaps would be calculated off that base index rather than the normal base index of 71.
DT