I had a CT9b fitted to my GT last year for a few months. I never got the chance to get it RR tested to see if there was a difference in the figures as it shit its pants. She did feel torquier in the low revs and not quite as pokey at the top end, but that is to be expected. The extra blade on the CT9b produces better low down torque which, as Jay quite rightly said, helps the EP91 get it's fat arse moving, at the sacrifice of the little loss at the top end.
I'd say a CT9b would get you better torque figures and slightly lower bhp. If you have a GT and you need to change your turbo and you're planning on keeping it stock, try and get your mits on the CT9b.