The “Off The Shelf” Lie

Scanspeak does not make drivers anything like those Richard Vandersteen manufactures himself, nor does anyone else. 

Danny Richie designed the planar-magnetic driver he incorporates in some of his speaker models, having it manufactured for GR Research. It is an improved version of the no-longer available NEO 3 tweeter. Scanspeak makes no such driver, and never has.

Both Magnepan and Eminent Technology manufacture their own planar/magnetic drivers, but p-m’s are not "normal" ("cone") drivers. I consider the ET LFT (Linear Field Transducer) driver superior to any and all cone drivers.

And then there are electrostatics, all of which are far more transparent than the vast majority of cone drivers. Scanspeak does not make an electrostatic driver, nor does SEAS.

I'm a ESL Speaker user for 30+ Years and have heard numerous ESL Speakers over the years, along with Panel Types Speakers from the main Brands.

As for Transparency they are without doubt the design that easily holds that mantle.

My most recent purchased Speakers of a Cabinet Design and with a non typical compliment of Cone Drivers have been through the quality on offer the successor of my ESL's.

The New Speakers are equal if not more Transparent than owned ESL's, but the design extends in making a impression with Room Coupling and coherence across the frequencies in a way the ESL's could not produce as a naked Speaker in my listening space. 

One major thing in favor of in house production is quality control. When the people who build the product work for the owner/designers directly, parts and quality issues can be identified and addressed quickly and easily. When you outsource, you provide specifications, but how those are achieved is at the discretion of another company whose employees answer to that owner. In the event of a bad batch, it becomes an adversarial issue instead of being resolved in house before manufacturing continues. 

Whether it’s speakers or other components, the ability to hand select parts from a batch or inspect and test each driver at each stage of manufacturing is what drives up costs but also quality. 

Sometimes marketing is a gimmick to raise the desirability of mediocre products but sometimes it’s a way of informing potential customers of why their products are worth the cost. 

Neither the OP nor any of the commenters seem to have mentioned that there's a third option, and that is the speaking is manufactured  by a third-party, such as Focal or Scanspeak, but modified (by the same OEM manufacturer) based on the speaker manufacturer's (such as Wilson) unique needs.

That can be a very valuable, not uncommon, and R&D- and cost-wise probably not a bad option.

The only thing that matters is whether the end sound of your system pleases you.

Some designers are known to turn sows' ears into silk purses. Andrew Jones comes to mind. It's a talent. Others can't help but turn silk purses into sows' ears.