From playing hide and seek vs. a Fiddlesticks I'd say CDR doesn't need to be any higher.
Also, CDR feeling underwheling is more a result of how it's represented as a statistic. If it were represented the same way as say, attack speed, 40% CDR equates to being able to cast abilities 66.67% faster. 50% CDR would allow you to cast abilities 100% more frequently though - each additional point of CDR you pick up is more powerful than the last, making it the only attribute that really scales exponentially with itself. I'm for more CDR options, but not for anything that would increase the cap beyond 40% reduced cooldown times.