All the GM electronically controlled transmissions (I can't remember the year the Bonneville got a fully electronic transmission, '90 or '91?) should shift into the next gear at redline, regardless of the position of the gear selector. Also they will not downshift into a gear until it is safe to do so (car has slowed down enough for the RPMs to drop below redline).
The main issue with manual shifting our cars is that running it to redline is often not the best for performance, because the powerband ends before it. If you are really interested on seeing the difference you can always take it to the track and run both ways, or get a GTECH or something similiar.
1992 Pontiac Bonneville SE, 163k miles - RIP (blew the transaxle)
1995 Honda Civic EX, 5psd