would be far to loud, ive got a Milltek and i had the decat but after a week i put the cat back on coz i couldnt be doing with the noise, much pref to hear the grunt from the engine rather than loud exhaust.
Not sure on performance, with mine the decat made it pick up more on low revs but i lost the kick at 5000rpm (prob coz its used the power already on the low revs??) would think it would be same with straight through center pipe.