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There is one instance here where the pyrex will beat the BK7 and that is in those winter nights when the temperature drops and just keeps dropping eg a Canberran winter night. A fan helps to keep the mirror close to the ambient but there will always be a difference. Enter the pyrex mirror with its better thermal expansion coefficient. This basically means that the mirror will distort less than a BK7 for a given temperature difference.
Most people will not even notice the difference but those doing high resolution astrophotography will get better performance from a pyrex mirror, under those conditions, than a BK7.
For us mere mortals a fan will do the job.
Cheers
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