They probably would not have blown the planes up over the Atlantic.
More likely they would have detonated their explosives over the large cities where the planes were bound and creating collateral damage on the ground and full press coverage.
Lockerbie looked and was more devastating and dramatic than floating debris in the middle of the ocean.
Even at that, the terrorists were going to be on planes during high occupancy periods.
Large planes with total passengers of around 200-300 each, times 20 planes plus ground casualties and the number could have been as high as 10,000.
They might have fallen on me and my wife. Seriously, we are under a downwind leg into a major international airport in California.