I have read the average lifespan of a geo system averages 22 years. I installed a ClimateMaster T27 last year - what is a reasonable life expectancy for this unit?

22 is as good a number as any, I suppose.

I often hear 15 years bandied about as average lifespan for airsource units. They have many of the same major components as geo, but the outdoors is hard on them.

I would guess that the inside air handler would last a lot longer, given it's not exposed to the elements.  If you can find a compatiable compressor replacement, the replacement cost would be considerably lower.

 

Compressors can be and often are replaced. Airsource outdoor units (commonly called condensers) incorporate several other components prone to failure before or about the same time as the compressor:

1) Accumulators rust out and leak
2) Reversing valves freeze in place
3) liquid line driers rust out and leak
4) Contactors weld closed

Any repair involving recovery or replacement of the refrigerant charge tends to run into several hundred dollars or more, supporting the choice of outright replacement