Your going to get about 10dB higher power output (assuming RMS, not peak, wattage), but it should be higher quality. The volume level increase depends on other factors (i.e., speaker sensitivity), but with the OEM speakers, you should get about twice the volume (if the new speakers are more sensitive, you'll get more).
If it were me, I'd run the ground to the OEM radio ground. If that's a chore, try to find an existing ground near the amp.