Sounds like a restriction in the fuel system, possible injector or contamination. Typically 7-8 is at the end of the fuel delivery before the return. I am not sure how Mclaren has their cylinder references noted. If; RT bank 1-4 and LF bank 5-8. The fuel supply looks to enter the right bank at the rear (4), front of motor at firewall, and exits at left bank rear (8) so 7 and 8 are at the end of the supply prior to the return.
Possible causes:
..restricted/clogged injector or bad injector.
..leak in the air intake, air box or air charge system
..contaminated MAF sensor
Under the described condition under load I would look closely at the fuel delivery. I would assume the factory test equipment has a data stream monitor. I would be interested to see what the long and short fuel trims would be on the scanner. It should also have the ability to show this data while driving when it happens and capture the error data to review. Years ago with Ford we had a "black box" that could be installed for intermittent issues where the customer depressed a button during the concern and it stored a log of data before and after the trigger.
If the data capture does not help I would swap the injectors to the suspect cylinders to two others and see if the faults follow the injectors. It does not look too involved to exchange as the rail sits between the valve covers and intake riser under the air charge/intake tubes.
David