For the record (and if it hasn’t changed), they will accept you at BOG before 8:25.
Taken from another Disney Blog:
“Also keep in mind that on a day with a regular 9am open, anyone with a Be Our Guest Restaurant reservation before 9am will be allowed to head back immediately after being released from the main entrance at 7:45am. If your reservation is for 8:30am or 8:45am then you don’t have to wait until then to head back to breakfast. Your reservation is just as good as an 8am reservation as far as Disney and the restaurant are concerned”.
So it could be better off arriving at 8:00 to have time for breakfast and be in front of the crowd at 8:45. Also note that pre-ordering your food will also speed things up.
For EMH at MK: If you want to RD 7DMT, I would arrive between 7:15 and 7:30. If not, 7:45 should be good.
For EMH at Epcot, you should be ahead of the crowd for TT, Soarin’ or FEA arriving at 7:30.
For EMH at AK, I would arrive at 7:00 to RD FOP. If not, 7:45 should be good.
For HS, I would arrive at 6:45 to RD SDD. If not, 7:45 should be good.
Hope it helps !