talking about stalls...
 

Anyone with a ball park figure on how much the stalling CAS will increase as one goes from seal level to 40,000 feet?? percent or number of knots?

Or a guess on how much the stalling aoa in degrees will decrease??

Isn't it handy that an aircraft will stall at (about) the same ias, no matter what the altitude or TAS?

Are you quite sure of that ?

Anyone with a ball park figure

Plenty of useful references on the net to review .. this Boeing article might be a good starting point -

Aero 12 - Angle of Attack

To echo some other comments, we may be sidetracking off the OP's question, which is really quite a high level one.

Also agree it depends on what you mean by "performance". Perhaps a starting point maybe to have a look at the theory behind "Flight Envelopes". This was my first introduction to performance theory back in my teens when started reading some discussion from Chuck Yeager I got a hold of on why aircraft "perform" differently at different altitudes
en.wikipedia.org/wiki/Flight_envelope

In terms of ability to accelerate, climb or maneuver, generally this is better at lower altitude (and worse at higher altitude) due to higher lift and higher excess power (Generally, although that can depend on the powerplant and airframe combination in question). These are probably the key "performance" characteristics to keep in mind when looking to fly the plane safely. Certainly, this has been topical in the spate of high-altitude stalls/incidents.

Now, in terms of speed, I tend to feel true "performance" is true airspeed. Yes IAS can be different, but on a discussion on aircraft performance...somewhat irrelevant unless we want to talk about a question separate from that the OP asked.

Maximum speed depends on the airframe and powerplant in question. Some planes are designed they will achieve this at near ground level, others will achieve it at higher altitudes.

Puts new meaning to...
 

Being in the dog house! Funny how we get ourselves into those situations!