i can say from experience that a snorkel turned towards the back will not cause that difference,
i would suspect:
poor wheel bearing torque after birfield replacement
head wind
contaminated blocking the air flow
other damage than just the birfield
another mechanical reason for the poor performance.
the vacumm created at 70 km/hr would be negligible let alone dropping a top speed 30 km/hr
well i did smash a cv that trip, so it and axle shaft was removed and everything else put pack in place. maybe wheel bearings not set up right, even brakes cause there was crap loads of mud on the trip. it was a side of the track job, in the rain of course