Well, I did some testing last night & here is what I found. This test could be considered a little subjective, or should I say could be skewed because I don't have a pile of batteries to test. I have a few newer AGM's but two of them are 400CCA batteries for the 4-tecs.
What I tested is two CB16 style that are of the same mfg, & both bought last spring about a month apart. They are distributed by Mills fleet farm in my area (a semi local chain store) I tired to figure out who makes them for fleet farm & I came up with Johnson controls & Deka. Maybe Racer can figure it out? they are private labeled "road runner" They are in the same price range as most for this size. $66 for a std 240cca & $85 for the 325cca AGM. nice part is they have a 1 year replacement vs 90 days of most brands.
OK, I think for this test to really hold any water, I would need at least two brand new batteries of each type, off the shelf instead of just one of each that were in service last season. So think of this test like holding a wet finger in the air to see what way the wind is blowing.....
I tested both batteries after being fully charged by the same 1.5A charger,
Using a stop watch & a multimeter I loaded both batteries to their rated capacities.
the std battery: loaded it at 240cca and after 15 seconds it showed 10.29 volts
the AGM battery: loaded it at 325cca and after 15 seconds it showed 10.75
I let them recharge again overnight & tested them both this morning at 325cca load
the std battery: 9.62volts
the AGM: 10.58
I am also sure there is some human error as I manually dialed in the load, hit the stop watch & locked the voltage. But in this case the AGM is stronger. I will save the info I have & post more results if I get the chance. Only problem is I am a little AGM brainwashed now, so I am not so sure I will have a fresh std battery to test again.
It would have been nice to see the std battery make a stronger initial test at its rating over the AGM, or at least equil to make me feel feel both batteries where up to par. But after the initial test, it would seem maybe the std battery was a little weaker? Really need more batteries to accurately figure it out IMO.
Really tying to be unbiased, so if I seem wishy washy that's why......