Do speaker makers customize drivers?

Yes speaker makers do have custom mods to drivers. they buy in bulk for thier models
It all depends on the speaker maker.
The custom unit may have a different coil, different Q different impedance.. all sorts of minor variations could be specified.
If the company driver is $300 MORE than direct, and your speakers are less than $3,00. I would go for a PAIR of the cheaper direct ones.
If these are expensive speakers, and you are taking about lesst than $200 more buy the speaker co driver. In fact buy a pair of them, so both cabinets have replacement drivers.

You'd have a better shot if you told us what speakers you own and the model number of the driver you think it is. Many use off the shelf, some have custom ones made.

If the driver is special to you speakers I would talk with Bill Legall of Millersound.

Millersound
1422 Taylor Road
Lansdale, PA 19446-1531
(215) 412-7700