You can focus on this aspect too much. Bikes are very efficient. I recall reading an article in Scientific American about Bicycle Physics years ago. They had the then standard professional racing bike. With their standard rider (a certain weight and certain horse power), the bike went something like 32 mph. The most tricked out bike available at that time went 33 mph with the standard rider. The perfect bike which was weightless, frictionless and had no wind resistance went 34 mph.
The biggest limit on bicycle top end speed is wind resistance and most of that is caused by the rider. Putting the rider into a faring and prone is about the best thing to do, but it does not seem to be very practical for riding on the road.
Some years back, I used to ride with a fast group. Almost every bike in the group was Dura Ace with a few Campys. I was riding 105 which was viewed with disdain. The guy who was easily the strongest rider in the bunch was riding Tiagra.
Training and keeping your weight down will have far greater effects then spending $$$$$$$$ on the bike.
Now if you are an elite cyclist and are competing against other elite cyclists, the small gain you get from a really high end bike is well worth it. I don't bother because I sort of suspect that if I have not made the Tour de France by now, it ain't happenin.