An JAB that has been well tailored to you by a good tailor will look far better than an Zegna OTR that hasn't been tailored to you (unless you happen to be one of the lucky few whose bode is a perfect match for Zegna's pattern, which is possible but unlikely).
However, if you want a tailor to turn a JAB into a Zegna, he will be unable to do so. He may be able to get the waist suppression to be similar, but details such as the shoulder and lapel will be impossible to modify to be Zegna-like. Not to mention the fabric and stitching quality and half or full canvassing.
So, to sum it up, if you want a Zegna, you'll need to get Zegna (or a good Roman/Neapolitan bespoke maker with a similar style and access to Zegna cloth). If you want a good fit, buy JAB (or BB, or HSM, or whatever) and spend some money at a good tailor.