CO2 Reactor Efficiency

I have a Sandpoint CO2 setup for my 125 gal planted tank.  Starting with
RO/DI water, I use a carbonate buffer (Kent PH Stable) to obtain 2-3 dKH. 
(My current fertilizer scheme is SeaChem Flourish, K2SO4, KNO3 and Epsom
salts with Dupla laterite and Flourish tabs in the substrate.)

Finally, I think I understand the KH/pH/CO2 relationship (thanks to George
Booth and Paul Sears' many postings).  A few days ago it dawned on me that
my CO2 levels were quite low (4-5 mg/l) based on my pH and KH readings.  So
I cranked up the Sandpoint about as far as it could go (many bubbles per
second, swirling of bubbles at the top of the reactor, with just a tiny bit
of direct loss of CO2 from the top of the reactor). 

The pH dropped nicely from 7.2 to 6.8 (Pinpoint pH monitor) with some daily
fluctuation of .3 or so.  With 2.5 dKH and a pH of 6.8, the CO2 level MUST
be about 12 mg/l, unless some of the stuff I'm adding to the water is
resulting in bad KH readings.  Right?

My question is:  Should I be thinking about a more efficient reactor? 
Would a Dupla "S" or Aqualine Buschke reactor allow me to reach the 15-20
mg/l range?

For those of you who have mastered this hobby (and I'm not in this group)
what CO2 levels made a difference for your plants?  Is 8-12 mg/l enough? 
(I've got 160w of florescent lighting, 60w of compact fluorescent and may
add as much as another 200w of CF lighting in the next month or so.)

TIA for any responses,  Steve Dixon