Depends on what type of boost and what type of racing (if any) you do.
For a centri type setup I would prefer the more aggressive gearing of the 10 speed. For a roots/twin screw setup you would probably be better off with the longer gearing of the 6 speed.
Wouldn't really want to consider a 10r80 swap. Looking to get a different car. I like the 10r80 but not a ton of data out there yet on how they hold up to power.
i think it was around 800whp + they said the clutches started slipping in 7th. if buying new id go 18, unless you come across a good deal like a boosted auto or an RS3. but im happy with the a10. if you want big boy numbers id say go 15-17 car. if you good around 600-750whp goal then 18 seems solid. theyre booming NA