New staircase? Are you just saying it or are you planning to add it? 
It has already been added...download the GSX Vehicles Update 3, and have a look at it.
Have you tested the aircraft with GSX yourself?
Obviously yes.
Because a have watch it closely and the stair did extend up to 2.17 as I set the point to be
As I've said, since the minimum height the stair can go is 2.22 mt., if you have a door set at 2.17 it won't even show up.
But if you are seeing it, with the door at 2.17, and you are seeing it extending, it means a very simple thing: your airplane is very LIGHT, so the door is NOT at 2.17 anymore!
If the airplane is lighter, the door might have gone higher than 2.22. As you say in the next message, it can go as high as 2.70. In that case, the stair that can go from 2.22 and up WILL finally show, and it WILL extend from 2.22 to 2.7. That's entirely NORMAL and LOGICAL.
The only "issue", up to today, was that GSX didn't had any smaller stair model that could go lower than 2.22 mt. Such model was planned long ago, so I finally added it following your suggestion.
It's a simpler model that can go from 1.87 to 2.22 so it can service all the AVRO/BAE series, and of course it would be nicer if we had a human pushing, but for the time being, it will serve its purpose.