i would not be concerned at all. A no muffler system is far better for turbo cars than one with a muffler...if you don't believe me just go to any supra or rx-7 board and check out what they are saying. These cars are turbo and will smoke a pt any day of the week whether we like it or not.
As for performance gains, i would imagine that a good exhaust will yield decent hp and ft/lb gains, but i won't estimate gains, i'll wait for the dyno sheets. What i can tell you is that turbo cars w/ a good full exhaust on average will gain between 15 and up to 30 hp at the wheels. Keep in mind that a 30hp gain is usually seen on twin setups...there's no way we've gained that much. Having a good setup is a key building block to future high performance gains