Regarding the learning curve: not really a problem. We give everybody copies of "Practical Common Lisp" and they learn it quickly. This is partly because we only hire excellent programmers. I like working at ITA so much because the people I work with are so great. I learn new things every day.
Common Lisp is a multi-paradigm language, supporting procedural, functional, object-oriented, etc. programming. Do we use higher-order functions? Sure. Do we use object-oriented programming? Yes, very heavily; lots of CLOS, a home-grown object-relational data mapping system based on CLOS, etc. Does our code look like the Scheme code in "Structure and Interpretation of Programming Languages?" No. We get lots of mileage our of Lisp macros; as I understand it, so does QPX.