Looking for your inputs.  What has been your experience with respect to manifold pressure at altitudes above 20,000 feet?  

Are you able to keep maximum MAP all the way up, and what is your critical altitude where you start to see a drop?  

And how high do you fly?  

And lastly, what cruise settings do you use once you get up there?

I paid extra money for upgraded turbo's (from a Mirage, which have bigger turbine wheels) in my engine rebuild, and want to see what my expectations should be based on your experiences.  If you happen to have a TSIO-550-X from Barrett that has this modification I would be especially interested in your experience up high.  I fly long distance (> 1100 miles non stop) routinely and would love to get in the FL250 range.  Thanks for your inputs -