2. I often fly into/out of Big Bear City, Ca. (elev. 6500ft.) I've been able
to get out lightly loaded (me and one in the right seat) at a density altitude
of 8,100 with one of our clubs 172s with the 320H2AD engines at 160 hp. What
difference would I see with the 15 fewer horses up front?

Don't know if you're partial to 172s for any reason, but you may want
to open up the search to other aircraft. Piper and Grumman may offer
you something with suitable performance and a slightly less expensive
purchase price.

Are 180hp (less expensive ^ ) Grumman's really suitable for frequent
8000+' DA takeoffs ?