Domino XL cutter options

[ear3]

Are there any other companies besides Seneca that make a 6mm cutter for the Domino XL?  I was looking to make extra deep 6mm mortises, which is why I would need an XL specific cutter (as opposed to doing it on the DF500 or using the RTS adapter).  $95 is pretty steep for a domino bit, though. 
https://www.senecawoodworking.com/products/6mm-cutter-for-domino-xl

 

[Tyler Ernsberger]

I haven't seen any other bits.  I remember the reason for the high price is that Seneca has to buy the Festool 8mm bit and machine it down to 6mm. 

 

[Holmz]

How do deeper mortices help?
How does one test the strength to show that they help?

 

[ear3]

Oh well. I guess I'll bite the bullet and get the bit then.