48v bank (8xGC2) same set since 2008
On weight it was basically a trade off for me, batteries in tray where old engine was, boat balanced the same.
A GC2 battery weighs about 65lbs. That makes 8 of them about 520lbs. The stock engine in your O'Day 28 appears to have been a Universal 12, which weighs 175lbs. I wouldn't call 350lbs extra "nothing", especially since the battery and other parts of the system aren't zero pounds either.
Since you have the monitor it would be interesting to know how many watts you are putting back into the battery bank when sailing at a typical speed (say 5 knots).
There is one user report here:
THE BIANKA LOG BLOG: ANOTHER ADVANTAGE OF ELECTRIC PROPULSION: REGEN
Looking at the video it looks like he is generating an average of 0.5 amps at 6 knots. He links to another video which appears to generate an average of about 1-2 amps at over 7 knots. Neither video reports voltage, but I'll guess that is measured 48 volts.
On this page you can see the power consumption for motoring:
THE BIANKA LOG BLOG: HARBOR TEST 2013 Part Two: Watts Up With The Data!
So if you motor this boat for 10 minutes at 4 knots (30 amps) you will consume 4.5 amp hours. If the regen system can put back in 0.5 amps when sailing at 6 knots you would need to sail for 9 hours to top off the battery banks completely.
Does this match your experience?