There is nothing wrong with going to 51k if you understand what you are doing.
I used to brief my new copilots thoroughly on aerodynamics and physiological effects. Then when conditions were right take them up there at least once and fly it manually. Then they were not affraid, understood it and if they ever had to do it for real it would not be their first time.
I am always afraid of the base experience being diluted because pilots do not see situations when they are in the RHS. Then the first time they do see a given situation is when they are a captain with a new co in the RHS. This can be carefully and safely prepared for by proper training.
I once worked for a company, not too long ago, where we sent a new F/O through the yellow submarine training just because we thought it was a good idea.
MM