6 dB over a 1/4-wave whip is 8.14 dBi.
The 1/4-wave whip has a gain of 2.14 dBi .......


The half wave vertical should have that 2.14 dbi gain.
A 1/4 wave ground plane should show about .3 db less...
Or when modeled anyway....

In general, a properly designed dual 5/8 collinear should
have appx 5.1 dbi gain. Or appx 3 dbd.... And yes, that
is slightly more than the dual 1/2 collinear should do.
If a single half wave shows 2.15 dbi, I wonder how adding
a second element can add another 3.85 db gain...???
Myself, I think their gain numbers are a little happy...
Also, in that super J article, the 1/4 wave and 5/8's they
compare to are mobile whips...This would greatly skew any
comparisons with the super J, being it's not a mobile antenna.
IE: make it look better than it really is...
An average elevated 1/4 GP will show appx
1.8 dbi or so...Only about .3 db difference from a 1/2 wave
that is elevated.
Of course, in the real world, the results can vary greatly
due to decoupling, or the lack of, etc...
I'd have to check/model the gain of the super J, but I'm
almost sure it would be less than 5.1 dbi...I'll take a wild
guess and say maybe .6 db less??? MK