Welcome to CAC!
Bosch spark plugs work just fine in a corvette. I just put Bosch Platinum +4 spark plugs in mine. They work great.
Did your daughter's mechanic replace the spark plug wires when he changed the plugs? Most of the time the wires will go bad long before the spark plugs. This is usually indicated by the engine "skipping"(running really rough & shaking) because the spark plug wire(s) aren't conducting the spark to the plug and not igniting the fuel/air charge in one or more of the cylinders.
If the wires are overlapped or touching they can also short out and the spark destined for the one cylinder can jump onto another cylinders wire. You can check for this by raising the hood and running the car in a darkened garage. You'll see sparks jumping around the wires.
If the engine is running smoothly but seems underpowered then you could have a restriction either in the engine air intake system or in the exhaust system.
If it's in the intake system you could have a stopped up air filter.
My car developed a problem like yours once after I ran over a large chunk of cement pavement from a pothole on I-75. It put a big dent in the exhaust pipe just before the catalytic converter. It severely restricted the flow of exhaust and choked the engine's performance. The y pipe had to be replaced to fix the problem.
Another thing that can happen is that the catalyst element in the 2 precatalytic converters or the main catalytic converter can get stopped up from running leaded gas or gasoline thats been contaminated by something else that reacts with the catalyst. Sometimes the catalyst will break loose, either due to age or an impact, from the inside the the converter and clog the exhaust pipe.
That's the most common stuff I know of that can cause you mileage to nosedive. Of course, a lead right foot will also cause poor mileage and speeding tickets.
Your problem could be something else. These are pretty complicated vehicles. I strongly suggest you find a reputable
Corvette mechanic in your area. Dealership mechanics very rarely fall into this catagory. You can find one by asking around a local corvette club, in one of the corvette specific magazines, or right here in the CAC forums.
good luck!
