General - I own many Benchmades (including M2 710) and many CRK knives, and customs. I have to say that the Benchmades are not very consistent at all in quality of manufacture: poor edge geometry, inconsistent grind lines, excessive heat treating, etc., etc. But I do like my Axis lock, and carry it alternately with my Sebenzas and Microtech LCC. By the way, most 710s I have handled seem to be very well made.
With proper edge geometry, my 710 is functionally the equal of my Sebenzas.
But there is the difference. Sebenzas are consistently as flawless in manufacturing as any knife on the planet. Pick one up here, pick one up there - they are all the same and at a uniformly near-flawless construction. As the late James Mattis noted, the difference between a Pinnacle and a Sebenza is all in the details.
And the very rare times when a Sebenza isn't quite right - well, CRK customer service is at the highest level, bar none.
Are true customs any better than the Sebenzas? In my opinion, no - not in terms of flawless construction, materials, or consistency. Flawless is flawless.
Where customs are better than Sebenzas are in matters of choice: design and materials.
But I have purchased knives from some of the top custom makers in the world, and their execution in manufacture is no better than a CRK! And many times the custom maker's customer service/customer relations leaves much to be desired.
Bottom line: Sebenzas are at the top tier in the industry in consistent quality, materials, and customer service. They are worth the money. But I cannot argue with you if you find a custom maker that can make a knife to your specification with your choice of materials and design, and build it to perfection, and give you superb after sales service. If you can find all that in a custom knife, then go for it! (But I sincerely doubt you will get all that for $350!)