After serious thinking Andrew Gabriel wrote :
Just to point out... if you're drilling through the glass and it's
toughened glass (as increasingly they are nowadays), then you won't
get as far as drilling the second hole. You bypass that and go
directly to the dustpan and brush stage, followed by the yellow
pages ;-)

You do NOT drill the glass, you drill holes through the gap between the
panes (through the seal).

To answer the other question about a repaired panels insulation
properties, I would imagine it will be similar to when the panel was
new - why would it be any different?

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk