I've had this discussion with my guitarist several times. His theory is, he sees himself as an average guitarist, so throwing loads of money at gear will make him sound better. My arguement is, that it would be imperceptible to 99 percent of the people listening. However, if he perceives himself as sounding better, then he will play better. Therefore, yes, expensive gear can make you sound / play better. But in my opinion it's mostly a placebo effect.