I made a similar imp stout about 6 weeks ago and used 2 packets of yeast for 2 reasons. Firstly the high gravity of 1.080 and secondly because I have had issues in the past with only using one packet and not achieving the desired FG, even after a week of fermenting.
That was the first time I tried using two packets and it fermented in 2 days.
I did the same a couple of weeks after that and the batch fermented in 2 days again.
So getting the job done in 2 days is very appealing to me but I never expected that amount of pressure build up in the FV.
I've only seen that once before, years ago and not as severe, when I used beer enhancer instead of dextrose in a Cooper's 40 pint stout kit.