I know you're fairly entrenched in the 13B approach, but for the sake of

discussion, I'm not sure that's a valid reason.

How high do you want to go? I think Tracy's been over 15k with an NA

13B. I'd be willing to bet that a Velocity with an NA 20B could get well

over 18k without a problem.

 

As I recall my analysis those many years ago; the cross over point on power for a turbo-normalized 13B and a NA 20B is at about 13,500’ msl.  Above that altitude the torbo 13B will do better.  Of course you can boost the 13B to more than 30” and get more power.

 

Al