I'm pretty sure these guys are talking frame only.
A complete chassis, swinger, arms, stem, subframe, can be 7K plus real easy.
I was over 8k on my Laeger new.
Can you take a closer pic where the two springs meet, at the cross over?
The top spring may be too soft, but you're kinda getting ahead of yourself until the engine is installed, the bike is complete and you have your ride heights front and rear dialed in.
Pics?
When your suspension is fully extended, how much pressure is being applied to the top spring?
Have you dialed in and setup your correct ride height front and rear?
1: I already had that crank from my previous build, which was a billet unit from Crank Works.
2: My understanding is 5 mil is the max you can go before having to trench the cases.
3: I felt 5 mil would be just right for my riding applications.
I noticed a considerable difference in timing changes from +4 to +7 on my old motor. Stock cylinder, 5 mil crank, 66 mil bores on VP C12. Duncan 403 Eliminator motor.
I will be finding out later this summer how the differences feel on my 5 mil Redline Serval.
I will be curious on your feed back once yours is up and running.
On my stock cylinder Duncan 403 motor I went from 15/40 gearing with 18" rears to 16/40. I was very happy with the difference making this change for track riding for a lot of the same reasons you have found. I was able to reduce gear changes quite a bit and rely on the motor further.
I don't think you have to use nerfs specifically for a banshee to make your needs/desires happen.
When I built my Laeger I ended up with nerfs that fit a YFZ 450.
The foot peg spud on my Laeger is similar to a relocated oem foot peg.
I would urge you to call several site sponsor builders and kinda interview them.
Finding the right builder who you can build a mutual relationship is key to you experiencing long term success with your motor.
Service after the sale is huge.