I've run open header before... and it rattles the inside of the car apart... not fun cruising around. I plan on buying some titanium pipe and making myself an exhaust system... I priced it out and it shouldn't be more than $300-350... which is less than most aftermarket exhaust systems.
I was thinking of having an exhaust bypass valve to run it through a pipe into the interior and under the seats so I could warm my arse up in the winter.
I ran an open downpipe on my turbo setup a few times at the track. Even at the track and with turbo (which muffles the exhaust some) it was still loud and embarassing.
Just for ****s and giggles you can disconnect your cat and drive around the block, you will see how much it sucks. lol
It's amazing how well mufflers and resonators actually kill engine noise. You really can't comprehend the noise level. Think Harley and add 10dB (double spl).