I'm running a QTP right now. It takes about 5-6 seconds from full closed to full open. Ground clearance is also an issue. But, like everything, there's always a compromise. I have a completely stock exhaust system from the converter back. Nice quiet cruising at freeway speeds. Looks completely stock. Sounds totally stock, even with the 272s.
Another issue you run into is tuning. Volumetric efficiency is different with it open and closed. At WOT with the cutout closed I run a full point richer. I also get compressor surge due to the backpressure.
So, there's good and bad to it. The compromises are worth it to me.