05 coupe with bose setup ? contemplating whether to upgrade my stock setup if it makes a big sound difference .... i personally dont need subs and amps and all that stuff

I put in a new Sony system and it not only sounds better than the stock HU but it was much more about the gain in features such as Pandora, Bluetooth Calling, Bluetooth Audio, AUX Jack, Navigation, etc.

I installed a Pioneeer HU last week AVH-X2500BT and it's more powerful than the stock HU. Now I feel that I might have to replace the speakers if I want to crank the volume up!

Did you guys plug in the PAC ROEM-NIS2 harness or did you connect the cables directly through a standard harness to notice a better sound?

I used the PAC ROEM-NIS2.

I think the Pioneer HU is more powerful than the stock Bose.
I'm with Kerry Garrison about the bells and wistles you have with newer HU. It looks better, it sounds better, mine has bluetooth for phone and music streaming, and I added a backup camera.

The short answer is YES!!! To add to it, if you purchase a radio with a good EQ you will notice the difference. The stock unit only allows you to minimally control base/treble but with an EQ you can customize each range. If you want pics of what a decent EQ looks like let me know and I'll reply with pics. (at work can't post atm)