It will help with flow but it doesn't sound good, but that also may depend on the exact glasspack you get,and your tolerance for 4 cyl rasp. I tried the 26" Thrush glasspack resonator and it may as well have been a straight pipe. Lots of harsh, raspy 4 cyl exhaust sounds made it through even the stock muffler. It sounded like a ricer on steroids with the thrush 2 chamber muffler...and so loud I had to swap the stock muffler on the next day (daily driver). I did run a punched out stock cat for a few months before that and it didn't increase the sound much with the stock muffler, but did sound slightly annoying for daily use with the 2 chamber. Keep in mind that changing the backpressure while running the stock chip can cause loss of low and mid range torque, which is what these engines really do best. The 944MAX chip I run is tuned for a test pipe, so I did notice some slight gains when running the empty cat and the glasspack, but the higher flow 2 chamber muffler I switched to seems to make it run better than stock even with the stock cat I am currently running.
The downside to a punched out cat is the weird pressure wave resonances that can get going inside the empty chamber and mess with the exhaust flow ( Good video explaination of this phenomenon here at 4:30
http://www.youtube.com/watch?v=Wa0BJqTG22Y&feature=channel_video_title ). The fiberglass packing in a glasspack burns out over time, especially with the cheaper brands. The factory cat is about half cat bricks, half glasspack resonator, so between the steel mesh liners left behind that support the cat bricks in the shell and the glasspack section, the hollowed out cat does a decent job of keeping the sound under control. The 944 motor is very large for a 4 cyl, being a 2.5L, and does make lots of undesirable 4 cyl type sound that has to be quieted down.
Play this vid in 720 HD mode for the best sound and you can hear some of the results of my last project. I do need to record how it sounds with the 2 chamber and the stock cat, ans I think it is the best of all of these. Skip to the 4:00 mark to get the audio answer to your question.
YouTube - Porsche 924S Thrush exhaust project