Pyrex vs BK7 may not be a major issue - there is certainly a difference, but see
http://www.skyandtelescope.com/howto...tml?page=2&c=y
The 16 inch GSO mirrors can be pretty good (not premium quality but good commercial quality) - see for example
http://www.astro-foren.de/showthread...r-im-Vergleich
others have reported less favourable outcomes on these forums, but I cannot find the posts
I have a 12 inch with a BK7 mirror - the focus wanders around a little bit if the temperature changes a lot, but that would happen anyway due to tube/strut expansion/contraction, regardless of the mirror material. The figure of the mirror does not seem to vary too much with normal temperature change rates.