VGA to CGA converters merely downscale/downsample the existing resolution. The resulting picture won't be as clear as native.
An ArcadeVGA is definitely the easiest way to get native arcade modelines. You can also use software like Soft15KHz
, although that won't protect you from 31KHz modes during bootup (not without a J-Pac).
The J-Pac is just a Jamma connector on an I-Pac. There's nothing stopping you using a JAMMA harness attached to your new chassis and an J-Pac (essentially building your own JAMMA arcade cabinet).
Boiling it down:
ArcadeVGA + I-Pac = 15KHz safe
J-Pac + Soft15KHz = 15KHz safe
I-Pac + Soft15KHz = potentially not 15KHz safe (depending on the arcade chassis from Jomac, and what sort of automatic cut-out it does or does not have).