I am the owner of an 88 740T. Recently I had 2 cylinders stop firing for different reasons. #2 had a failure in the ballast for the injectors and #4 had no spark due to an earthed lead. Both of those problems have been fixed and now it seems to be not produceing full power output. When accelerating I seem to be getting a strange buzzing noise coming through the firewall and the exhaust note has become much deeper and louder. The car already runs a 2.5" straight through exhaust with only 1 muffler so it is noticably louder. I had increased the boost via a controller previously to 11 psi and full boost is achieved though a hissing noise can be heard in boost and buzzing noise can be heard under the hood at idle from the inlet side. The aftermarket boost guage is showing only 18" mercury of vacuum at idle instead of the usual 21".
The most confusing thing about how the car is behaving is that none of these things were happening before the 2 cylinders stopped working. Does anyone have any ideas of where I should start looking, if so some help would be greatly appreciated.

Evan in Australia