Yes and no on the first question. If you're smart about it the car will have no problems with 27psi. If you lug a high gear you will run into problems...ie don't floor the car at 3k in 4th gear or 5th.

The 24psi tune tapering off is normal...it's the small turbo running out of steam with the airflow demand the engine wants.

 

Yes, you should be concerned. If you plan on using your car as a daily driver, something is going to let go soon running that much boost. People on these boards have gone through pistons, clutches, transmission, etc running that kind of boost.
It is just a matter of time.

 

You don't need to run that high of boost but it wont hurt anything. The turbo is efficient over 30psi at that RPM range but as GM Tech's have said, over 24psi will score the walls.