I think most people go with the plus 4 because they are more common and cheaper. With a +6 and larger I am pretty sure you would have to go with a thicker and longer sleeve with a plate. This would also require a longer cam chain. For the power gains and reliability purposes, most people could not justify it.
I would think that the power would be hard to make use of as the head would require extensive work. Excessive porting would reduce air velocity and make it hard to have a power except on the top end. Most people's riding style would not benefit from this work.
A barker +6 is just about as simple as the barker +4, though I believe there are some piston skirt clearance issues with certain 102mm pistons. No special sleeves, wristpin location, cam chains, or spacers.
The +8 I believe uses an offset wristpin, no spacers, and stock length cam chain.
Kenz has crankworks do most of their work, Barker still does the big 660's in house from what I understand.
They are more difficult to find because there is less demand, not because there are no companies willing to make the parts. But the larger cranks are not cheap and most of the guys willing to spend that kind of money to build a bike with a 10mm over crank are probably working on raptor 700's or DS650s because those machines have more potential to make power than the 660 engine, although there are some competent 660 guys who might debate that.
Yea i fought the same problem in the car seen, I bought a LT1 camaro in '02, and the LS1's came out in '98. LT1s became less popular to the hotrod world, therefore the parts deminished. I couldnt afford to purchase a LS1 and already had a start on my LT1.